HOMAGE TO SANTA ROSALIA
or
WHY ARE THERE SO MANY KINDS OF ANIMALS?*
G. E. HUTCHINSON
Department of Zoology, Yale University, New Haven, Connecticut
When you did me the honor of asking me to fill your presidential chair,
I accepted perhaps without duly considering the duties of the president
of a society, founded largely to further the study of evolution, at the
close of the year that marks the centenary of Darwin and Walleye initial
presentation of the theory of natural selection. It seemed to me that most
of the significant aspects of modern evolutionary theory have come either
from geneticists, or from those heroic museum workers who suffering through
years of neglect, were able to establish about 20 years ago what has come
to be called the "new systematics." You had, however, chosen an
ecologist as your president and one of that school at times supposed to
study the environment without any relation to the organism.
A few months later I happened to be in Sicily. An early interest in zoogeography
and in aquatic insects led me to attempt to collect near Palermo, certain
species of water-bugs, of the genus Corixa, described a century ago by Fieber
and supposed to occur in the region, but never fully reinvestigated. It
is hard to find suitable localities in so highly cultivated a land scape
as the Concha d'Oro. Fortunately, I was driven up Monte Pellegrino, the
hill that rises to the west of the city, to admire the view. A little below
the summit, a church with a simple baroque facade stands in front of a cave
in the limestone of the hill. Here in the 16th century a stalactite encrusted
skeleton associated with a cross and twelve beads was discovered. Of this
skeleton nothing is certainly known save that it is that of Santa Rosalia,
a saint of whom little is reliably reported save that she seems to have
lived in the 12th century, that her skeleton was found in this cave, and
that she has been the chief patroness of Palermo ever since. Other limestone
caverns on Monte Pellegrino had yielded bones of extinct pleistocene Equus,
and on the walls of one of the rock shelters at the bottom of the hill there
are beautiful Gravettian engravings. Moreover, a small relic of the saint
that I saw in the treasury of the Cathedral of Monreale has a venerable
and
·Address of the President, American Society of Naturalists, delivered
at the annual meeting, Washington, D. C., December 30, 1958.
145
Reprinted from THE AMERICAN NATURALIST, VoI. XCIII, No. 870, May-June, 1959
Copyright, 1959, The American Society of Naturalists · All right reserved
146 THE AMERICAN NATURALIST
petrified appearance, as might be expected. Nothing in her history being
known to the contrary, perhaps for the moment we may take Santa Rosalia
as the patroness of evolutionary studies, for just below the sanctuary,
fed no doubt by the water that percolates through the limestone cracks of
the mountain, and which formed the sacred cave, lies a small artificial
pond, and when I could get to the pond a few weeks later, I got from it
a hint of what I was looking for.
Vast numbers of Corixidae were living in the water. At first I was rather
disappointed because every specimen of the larger of the two species present
was a female, and so lacking in most critical diagnostic features, while
both sexes of the second slightly smaller species were present in about
equal number. Examination of the material at leisure, and of the relevant
literature, has convinced me that the two species are the common European
C. punctata and C affinis, and that the peculiar Mediterranean species are
illusionary. The larger C. punctata was clearly at the end of its breeding
season, the smaller C. affinis was probably just beginning to breed. This
is the sort of observation that any naturalist can and does make all the
time. It was not until I asked myself why the larger species should breed
first, and then the more general question as to why there should be two
and not 20 or 200 species of the genus in the pond, that ideas suitable
to present to you began to emerge. These ideas finally prompted the very
general question as to why there are such an enormous number of animal species.
There are at the present time supposed to be (Muller and Campbell, 1954;
Hyman, 1955) about one million described species of animals. Of these about
three-quarters are insects, of which a quite disproportionately large number
are members of a single order, the Coleoptera.1 The marine fauna although
it has at its disposal a much greater area than has the terrestrial, lacks
this astonishing diversity (Thorson, 1958). If the insects are excluded,
it would seem to be more diverse. The proper answer to my initial question
would be to develop a theory at least predicting an order of magnitude for
the number of species of 106 rather than 108 or 104. This I certainly cannot
do. At most it is merely possible to point out some of the factors which
would have to be considered if such a theory was ever to be constructed.
Before developing my ideas I should like to say that I subscribe to the
view that the process of natural selection, coupled with isolation and later
mutual invasion of ranges leads to the evolution of sympatric species, which
at equilibrium occupy distinct niches, according to the Volterra-Gause principle.
The empirical reasons for adopting this view and the correlative view that
the boundaries of realized niches are set by competition are mainly in direct.
So far as niches may be defined in terms of food, the subject has been carefully
considered by Lack (1954). In general all the indirect evi-
1There is a story, possibly apocryphal, of the distinguished British biologist,
1. B. S. Haldane, who found himself in the company of a group of theologians.
On being asked what one could conclude as to the nature of the Creator from
a study of his creation, Haldane is said to have answered, "An inordinate
fondness for beetles . "
SANTA ROSALIA 147
dence is in accord with the view, which has the advantage of confirming
theoretical expectation. Most of the opinions that have been held to the
contrary appear to be due to misunderstandings and to loose formulation
of the problem (Hutchinson, 1958). In any study of evolutionary ecology,
food relations appear as one of the most important aspects of the system
of animate nature. There is quite obviously much more to living communities
than the raw dictum "eat or be eaten," but in order to understand
the higher intricacies of any ecological system, it is most easy to start
from this crudely simple point of view.
FOOD CHAINS
Animal ecologists frequently think in terms of food chains, of the form
in dividuals of species S1are eaten by tbose of S2, of S2by S3of S3by S4,etc.
In such a food chain Sl will ordinarily be some holophylic organism or
material derived from such organisms. The simplest case is that in which
we have a true predator chain in Odum's (1953) convenient terminology, in
which the lowest link is a green plant, the next a herbivorous animal, the
next a primary carnivore, the next a secondary carnivore, etc. A specially
important type of predator chain may be designated Eltonian, because in
recent years C. S. Elton (1927) has emphasized its widespread significance,
in which the predator at each level is larger and rarer than its prey. This
phenomenon was recognized much earlier, notably by A. R. Wallace in his
contribution to the 1858 communication to the Linnean Society of London.
In such a system we can make a theoretical guess of the order of magnitude
of the diversity that a single food chain can introduce into a community.
If we assume that in general 20 per cent of the energy passing through one
link can enter the next link in the chain, which is overgenerous (cf. Lindeman,
1942; Slobodkin in an unpublished study finds 13 per cent as a reason able
upper limit) and if we suppose that each predator has twice the mass, (or
1.26 the linear dimensions) of its prey, which is a very low estimate of
the size difference between links, the fifth animal link will have a population
of one ten thousandth (10-4) of the first, and the fiftieth animal link,
if there was one, a population of 10-4 the size of the first. Five animal
links are certainly possible, a few fairly clear cut cases having been in
fact recorded. If, however, we wanted 50 links, starting with a protozoan
or rotifer feeding on algae with a density of 106 cells per ml, we should
need a volume of 1026 cubic kilometers to accommodate on an average one
specimen of the ultimate predator, and this is vastly greater than the volume
of the world ocean. Clearly the Eltonian food-chain of itself cannot give
any great diversity, and the same is almost certainly true of the other
types of food chain, based on detritus feeding or on parasitism.
Natural selection
Before proceeding to a further consideration of diversity, it is, however,
desirable to consider the kinds of selective force that may operate on a
food chain, for this may limit the possible diversity.
148 THE AMERICAN NATURALIST
It is reasonably certain that natural selection will tend to maintain
the efficiency of transfer from one level to another at a maximum. Any increase
in the predatory efficiency of the nth link of a simple food chain will
how ever always increase the possibility of the extermination of the (n-
1)th link. If this occurs either the species constituting the nth link must
adapt itself to eating the (n - 2)th link or itself become extinct. Tbis
process will infact tend to shortening of food chains. A lengthening can
presumably occur most simply by the development of a new terminal carnivore
link, as its niche is by definition previously empty. In most cases this
is not likely to be easy. Tbe evolution of the whale-bone whales, which
at least in the case of Balaenoptera borealis, can feed largely on copepods
and so rank on occasions as primary carnivores (Bigelow, 1926), presumably
constitutes the most dramatic example of the shortening of a food chain.
Mechanical considerations would have prevented the evolution of a larger
rarer predator, until man developed essentially non-Eltonian methods of
hunting whales.
Effect of size
A second important limitation of the length of a food chain is due to
the fact that ordinarily animals change their size during free life. If
the terminal member of a chain were a fish that grew from say one cm to
150 cms in the course of an ordinary life, this size change would set a
limit by competition to the possible number of otherwise conceivable links
in the I-l50 cm range. At least in fishes this type of process (metaphoetesis)
may involve the smaller specimens belonging to links below the larger and
the chain length is thus lengthened, though under strong limitations, by
cannibalism.
We may next enquire into what determines the number of food chains in a
community. In part the answer is clear, though if we cease to be zoologists
and become biologists, the answer begs the question. Within certain limits,
tbe number of kinds of primary producers is certainly involved, because
many berbivorous animals are somewhat eclectic in their tastes and many
more limited by their size or by such structural adaptations for feeding
that they have been able to develop.
Effects of terrestrial plants
The extraordinary diversity of the terrestrial fauna, which is much greater
than that of the marine fauna, is clearly due largely to the diversity provided
by terrestrial plants. This diversity is actually two-fold. Firstly, since
terrestrial plants compete for light, they have tended to evolve into structures
growing into a gaseous medium of negligible buoyancy. This has led to the
formation of specialized supporting, photosynthetic, and reproductive structures
which inevitably differ in chemical and physical properties. The ancient
Danes and Irish are supposed to have eaten elm-bark, and sometimes sawdust,
in periods of stress, has been hydrolyzed to produce edible carbohydrate;
but usually man, the most omnivorous of all animals, has avoided
SANTA ROSALIA 149
almost all parts of trees except fruits as sources of food, though various
individual species of animals can deal with practically every tissue of
many arboreal species. A major source of terrestrial diversity was thus
introduced by the evolution of almost 200,000 species of flowering plants,
and the three quarters of a million insects supposedly known today are in
part a product of that diversity. But of itself merely providing five or
ten kinds of food of different consistencies and compositions does not get
us much further than the five or ten links of an Eltonian pyramid. On the
whole the problem still remains, but in the new form: why are there so many
kinds of plants? As a zoologist I do not want to attack that question directly,
I want to stick with animals, but also to get the answer. Since, however,
the plants are part of the general system of communities, any sufficiently
abstract properties of such communities are likely to be relevant to plants
as well as to herbivores and carnivores. It is, therefore, by being somewhat
abstract, though with concrete zoological details as examples, that I intend
to proceed.
INTERRELATIONS OF FOOD CHAINS
Biological communities do not consist of independent food chains, but
of food webs, of such a kind that an individual at any level (corresponding
to a link in a single chain) can use some but not all of the food provided
by species in the levels below it. It has long been realized that the
presence of two species at any level, either of which can be eaten by a
predator at a level above, but which may differ in palatability, ease of
capture or seasonal and local abundance, may provide alternative foods for
the predator. The predator, therefore, will neither become extinct itself
nor exterminate its usual prey, when for any reason, not dependent on prey-predator
relationships, the usual prey happens to be abnormally scarce. This aspect
of complicated food webs has been stressed by many ecologists, of whom the
Chicago school as represented by Allee, Emerson, Park, Park and Schmidt
(1949), Odum (1953) and Elton (1958), may in particular be mentioned. Recently
MacArthur (1955) using an ingenious but simple application of information
theory has generalized the points of view of earlier workers by providing
a formal proof of the increase in stability of a community as the number
of links in its food web increases.
MacArthur concludes that in the evolution of a natural community two partly
antagonistic processes are occurring. More efficient species will re place
less efficient species, but more stable communities will outlast less stable
communities. In the process of community formation, the entry of a new species
may involve one of three possibilities. It may completely dis place an old
species. This of itself does not necessarily change the stability, though
it may do so if the new species inherently has a more stable population
(cf. Slobodkin, 1956) than the old. Secondly, it may occupy an unfilled
niche, which may, by providing new partially independent links, increase
stability. Thirdly, it may partition a niche with a pre-esisting species.
Elton (1958) in a fascinating work largely devoted to the fate of species
accidentally or purposefully introduced by man, concludes that in very
150 THE AMERICAN NATURALIST
diverse communities such introductions are difficult. Early in the history
of a community we may suppose many niches will be empty and invasion will
proceed easily; as the community becomes more diversified, the process will
be progressively more difficult. Sometimes an extremely successful invader
may oust a species but add little or nothing to stability, at other times
the invader by some specialization will be able to compete successfully
for the marginal parts of a niche. In all cases it is probable that invasion
is most likely when one or more species happen to be fluctuating and are
under represented at a given moment. As the communities build up, these
opportunities will get progressively rarer. In this way a complex community
containing some highly specialized species is constructed asymptotically.
Modern ecological theory therefore appears to answer our initial question
at least partially by saying that there is a great diversity of organisms
because communities of many diversified organisms are better able to persist
than are communities of fewer less diversified organisms. Even though the
entry of an invader which takes over part of a niche will lead to the reduction
in the average population of the species originally present, it will also
lead to an increase in stability reducing the risk of the original population
being at times underrepresented to a dangerous degree. In this way loss
of some niche space may be compensated by reduction in the amplitude of
fluctuations in a way that can be advantageous to both species. The process
however appears likely to be asymptotic and we have now to consider what
sets the asymptote, or in simpler words why are there not more different
kinds of animals?
LIMITATION OF DIVERSITY
It is first obvious that the processes of evolution of communities must
be under various sorts of external control, and that in some cases such
control limits the possible diversity. Several investigators, notably Odum
(1953) and MacArthur (1955), have pointed out that the more or less cyclical
oscillations observed in arctic and boreal fauna may be due in part to the
communities not being sufficiently complex to damp out oscillations. It
is certain that the fauna of any such region is qualitatively poorer than
that of warm temperate and tropical areas of comparable effective precipitation.
It is probably considered to be intuitively obvious that this should be
so, but on analysis the obviousness tends to disappear. If we can have one
or two species of a large family adapted to the rigors of Arctic existence,
why can we not have more? It is reasonable to suppose that the total biomass
may be involved. If the fundamental productivity of an area is limited by
a short growing season to such a degree that the total biomass is less than
under more favorable conditions, then the rarer species in a community may
be so rare that they do not exist. It is also probable that certain absolute
limitations on growth-forms of plants, such as those that make the development
of forest impossible above a certain latitude, may in so acting, severely
limit the number of niches. Dr. Robert MacArthur points out that the tevelopment
of high tropical rain forest increases the bird fauna more than that of
mam-
SANTA ROSALIA 151
mals, and Thorson (1957) likewise has shown that the so-called infauna show
no increase of species toward the tropics while the marine epifauna becomes
more diversified. The importance of this aspect of the plant or animal substratum,
which depends largely on the length of the growing season and other aspects
of productivity is related to that of the environmental mosaic discussed
later.
We may also inquire, but at present cannot obtain any likely answer, whether
the arctic fauna is not itself too young to have achieved its maximum diversity.
Finally, the continual occurrence of catastrophes, as Wynne Edwards (1952)
has emphasized, may keep the arctic terrestrial community in a state of
perennial though stunted youth.
Closely related to the problems of environmental rigor and stability, is
the question of the absolute size of the habitat that can be colonized.
Over much of western Europe there are three common species of small voles,
namely Microtus arvalis, M. agrestis and Clethrionomys glareolus. These
are sympatric but with somewhat different ecological preferences. In the
smaller islands off Britain and in the English channel, there is only one
case of two species co-occurring on an island, namely M. agrestis and Clethrionomys
on the island of Mull in the Inner Hebrides (Barrett-Hamilton and Hinton,
1911-1921). On the Orkneys the single species is M. orcadensis, which in
morphology and cytology is a well-differentiated ally of M. arvalis; a comparable
animal (M. sarnius) occurs on Guernsey. On most of the Scottish Islands
only subspecies of M. agrestis occur, but on Mull and Raasay, on the Welsh
island of Skomer, as well as on Jersey, races of Clethrionomys of somewhat
uncertain status are found. No voles have reached Ireland, presumably for
paleogeographic reasons, but they are also absent from a number of small
islands, notably Alderney and Sark. The last named island must have been
as well placed as Guemsey to receive Mi crotus arvalis. Still stranger
is the fact that although it could not have got to the Orkneys without entering
the mainland of Britain, no vole of the arvalis type now occurs in the latter
country. Cases of this sort may be perhaps explained by the lack of favorable
refuges in randomly distributed very unfavorable seasons or under special
kinds of competition. This explanation is very reasonable as an explanation
of the lack of Microtus on Sark, where it may have had difficulty in competing
with Rattus rattus in a small area. It would be stretching one's credulity
to suppose that the area of Great Britain is too small to permit the existence
of two sympatric species of Microtus, but no other explanation seems to
have been proposed.
It is a matter of considerable interest that Lack (1942) studying the populations
of birds on some of these small British islands concluded that such populations
are often unstable, and that the few species present often occupied larger
niches than on the mainland in the presence of competitors. Such faunas
provide examples of communities held at an early stage in development because
there is not enough space for the evolution of a fuller and more stable
community.
l52 THE AMERICAN NATURALIST
NICHE REQUIREMENTS
The various evolutionary tendencies, notably metaphoetesis, which operate
on single food chains must operate equally on the food-web, but we also
have a new, if comparable, problem as to how much difference between two
species at the same level is needed to prevent them from occupying the same
niche. Where metric characters are involved we can gain some insight into
this estremely important problem by the study of what Brown and Wilson (1956)
have called character displacement or the divergence shown when two partly
allopatric species of comparable niche requirements become sympatric in
part of their range.
I have collected together a number of cases of mammals and birds which
appear to exhibit the phenomenon (table 1). These cases involve metric characters
related to the trophic apparatus, the length of the culmen in birds and
of the skull in mammals appearing to provide appropriate measures. here
the species co-occur, the ratio of the larger to the small form varies from
1.1 to 1.4, the mean ratio being 1.28 or roughly 1.3. This latter figure
may tentatively be used as an indication of the kind of difference necessary
to permit two species to co-occur in different niches but at the same level
of a food-web. In the case of the aquatic insects with which I began my
address, we have over most of Europe three very closely allied species of
Corixa, the largest punctata, being about 116 per cent longer than the middle
sized species macrocepbala, and 146 per cent longer than the small species
aflinis. In northwestern Europe there is a fourth species, C. dentipes,
as large as C. punctata and very similar in appearance. A single observation
(Brown, 1948) suggests that this is what I have elsewhere (Hutchinson, l9Sl)
termed a fugitive species, maintaining itself in the face of competition
mainly on account of greater mobility. According to Macan (1954) while both
affinis and macroccpbala may occur with punctata they nevrer are found with
each other, so that all three species never occur together. In the eastern
part of the range, macrocephala drops out, and punctata appears to have
a discontinuous distribution, being recorded as far east as Simla, but not
in southern Persia or Kashmir, where affinis occurs. In these eastem localities,
where it occurs by itself, affnis is larger and darker than in the west,
and superficially looks like macrocepbala (Hutchinson, 1940).
This case is very interesting because it looks as though character displacement
is occurring, but that the size differences between the three species are
just not great enough to allow them a11 to co-occur. Other characters than
size are in fact clearly involved in the separation, macrocepbala preferring
deeper water than affinis and the latter being more tolerant of brackish
conditions. It is also interesting because it calls attention to a marked
difference that must occur between hemimetabolous insects with annual life
cycles involving relativrely long growth periods, and birds or mammals in
which the period of growth in length is short and of a very special nature
compared with the total life span. In the latter, niche separation may be
possible merely through genetic size differences, while in a pair of ani-
154 THE AMERICAN NATURALIST
mals like C. punctata and C. affinis we need not only a size difference
but a seasonal one in reproduction; this is likely to be a rather complicated
matter. For the larger of two species always to be larger, it must never
breed later than the smaller one. I do not doubt that this is what was happening
in the pond on Monte Pellegrino, but have no idea how the difference is
achieved .
I want to emphasize the complexity of the adaptation necessary on the part
of two species inhabiting adjacent niches in a given biotope, as it probably
underlies a phenomenon which to some has appeared rather puzzling. MacArthur
(1957) has shown that in a sufficiently large bird fauna, in a uniform undisturbed
habitat, areas occupied by the different species appear to correspond to
the tandom non-overlapping fractionation of a plane or volume. Kohn (1959)
has found the same thing for the cone-shells (Conus) on the Hawaiian reefs.
This type of arrangement almost certainly implies such individual ant unpredictable
complexities in the determination of the niche boundaries, and so of the
actual areas colonized, that in any overall view, she process would appear
random. It is fairly obvious that in different types of community the divisibility
of niches will differ and so the degree of diversity that can be achieved.
The fine details of the process have not been atequately investigated, though
many data must already exist that could be organized to throw light on the
problem.
MOSAIC NATURE OF THE ENVIRONMENT
A final aspect of the limitation of possible diversity, and one that perhaps
is of greatest importance, concerns what may be called the mosaic nature
of the environment. Except perhaps in open water when only uniform quasi
horizontal surfaces are considered, every area colonized by organisms has
some local diversity. The significance of such local diversity depends very
largely on the size of the organisms under consideration. In another paper
MacArthur and I (Hutchinson and MacArthur, l9S9) have attempted a theoretical
formulation of this property of living communities and have pointed out
that even if we consider only the herbivorous level or only one of the carnivorous
levels, there are likely, above a certain lower limit of size, to be more
species of small or medium sized organisms than of large organisms. It is
difficult to go much beyond crude qualitative impressions in testing this
hypothesis, but we find that for mammal faunas, which contain such diverse
organisms that they may well be regarded as models of whole faunas, there
is a definite hint of the kind of theoretical distribution that we deduce.
In qualitative terms the phenomenon can be exemplified by any of the larger
species of ungulates which may require a number of different kinds of terrain
within their home ranges, any one of which types of terrain might be the
habitat of some small species. Most of the genera or even subfamilies of
very large terrestrial animals contain only one or two sympatric species.
In this connection I cannot refrain from pointing out the immense scientific
importance of obtaining a really full insight into the ecology of the large
mammals of Africa while they can still be stutied under natural conditions.
It is
SANTA ROSALIA l55
indeed quite possible that the results of studies on these wonderhl animals
would in long-range though purely practical terms pay for the establishment
of greater reservations and National Parks than at present exist.
In the passerine birds the occurrence of five or six closely related sympatric
species is a commonplace. In the mammal fauna of western Europe no genus
appears to contain more than four strictly sympatric species. In Britain
this number is not reached even by Mustela with three species, on the adjacent
parts of the continent there may be three sympatric shrews of the genus
Crocidura and in parts of Holland three of Microtus. In the same general
region there are genera of insects containing huntreds of species, as in
Athela in the Coleoptera and Dasyhelea in the Diptera Nematocera. The same
phenomenon will be encountered whenevrer any well-studied fauna is considered.
Irrespective of their position in a food chain, small size, by permitting
animals to become specialized to the conditions offered by small diversified
elements of the environmental mosaic, clearly makes possible a degree of
diversity quite unknown among groups of larger organisms.
We may, therefore, conclude that the reason why there are so many species
of animals is at least partly because a complex trophic organization of
a community is more stable than a simple one, but that limits are set by
the tentency of food chains to shorten or become blurred, by unfavorable
physical factors, by space, by the fineness of possible subdivision of niches,
and by those characters of the environmental mosaic which permit a greater
diversity of small than of large allied species.
CONCLUDING DISCUSSION
In conclusion I shoult like to point out three very general aspects of
the sort of process I have tescribed. One speculative approach to evolutionary
theory arises from some of these conclusions. ]ust as adaptative evolution
by natural selection is less easy in a small population of a species than
in a larger one, because the total pool of genetic variability is inevitably
less, so it is probable that a group containing many diversified species
will be able to seize new evolutionary opportunities more easily than an
undiversified group. There will be some limits to this process. Where large
size permits the development of a brain capable of much new learnt behavior,
the greater plasticity acquired by the individual species will offset the
disadvantage of the small number of allied species characteristic of groups
of large animals. Early during evolution the main process from the standpoint
of community structure was the filling of all the niche space potentially
available for producer and decomposer organisms and for herbivorous animals.
As the latter, and still more as carnivorous animals began to appear, the
persistence of more stable communities would imply splitting of niches previously
occupied by single species as the communities became more diverse. As this
process continued one would expect the overall rate of evolution to have
increased, as the increasing diversity increased the probability of the
existence of species preadapted to new and unusual niches. It is reasonable
to suppose that strong predation among macroscopic metzoa
I56 THE AMERCAN NATURALIST
did not begin until the late Precambrian, and that the appearance of powerful
predators led to the appearance of fossilizable skeletons. This seems the
only reasonable hypothesis, of those so far advanced, to account for the
relatively sudden appearance of several fossilizable groups in the Lower
Cambrian. The process of diversification would, according to this argument,
be somewhat autocatakinetic even without the increased stability that it
would produce; with the increase in stability it would be still more a self
inducing process, but one, as we have seen, with an upper limit. Part of
this upper limit is set by the impossibility of having many sympatric allied
species of large animals. These however are the animals that can pass from
primarily innate to highly modifiable behavior. From an evolutionary point
of view, once they have appeared, there is perhaps less need for diversity,
though from other points of view, as Elton (1958) has stressed in dealing
with human activities, the stability provded by diversity can be valuable
even to the most adaptable of all large animals. We may perhaps therefore
see in the process of evolution an increase in diversity at an increasing
rate till the early Paleozoic, by which time the familiar types of community
structure were established. There followed then a long period in which various
large and finally large-brained species be came dominant, and then a period
in which man has been reducing diversity by a rapidly increasing tendency
to cause extinction of supposedly unwanted species, ofen in an indiscriminate
manner. Finally we may hope for a limited reversal of this process when
man becomes aware of the value of diversity no less in an economic than
in an esthetic and scientific sense.
A second and much more metaphysical general point is perhaps worth a moment's
discussion. The evolution of biological communities, though each species
appears to fend for itself alone, produces integrated aggregates which increase
in stability. There is nothing mysterious about this; it follows from mathematical
theory and appears to be confirmed to some extent empirically. It is however
a phenomenon which also finds analogies in other fields in which a more
complex type of behavior, that we intuitively regard as higher, emerges
as the result of the interaction of less complex types of behavior, that
we call lower. The emergence of love as an antidote to aggression, as Lorenz
pictures the process, or the development of cooperation from various forms
of more or less inevitable gtoup behavior that Allee (1931) has stressed
are examples of this from the more complex types of biological systems .
In the ordinary sense of explanation in science, such phenomena are explicable.
The types of holistic philosophy which import ad hoc mysteries into science
whenever such a situation is met are obviously unnecessary. Yet perhaps
we may wonder whether the empirical fact that it is the nature of things
for this type of explicable emergence to occur is not something that itself
requires an explanation. Many objections can be raised to such a view; a
friendly organization of biologists could not occur in a universe in which
cooperative behavior was impossible and without your cooperation I could
not raise the problem. The question may in fact appear to certain
SANTA ROSALIA 157
types of philosophers not to be a real one, though I suspect such philosophers
in their desire to demonstrate how often people talk nonsense, may sometimes
show less ingenuity than would be desirable in finding some sense in such
questions. Even if the answer to such a question were positive, it might
not get us very far; to an existentialist, life would have merely provided
yet one more problem; students of Whitehead might be made happier, though
on the whole the obscurities of that great writer do not seem to generate
unhappiness; the religious philosophers would welcome a positive answer
but note that it told them nothing that they did not know before; Marxists
might merely say, "I told you so." In spite of this I suspect
that the question is worth raising, and that it could be phrased so as to
provide some sort of real dichotomy between alternatives; I therefore raise
it knowing that I cannot, and suspecting that at present others cannot,
provide an intellectually satisfying answer. My third general point is
less metaphysical, but not without interest. If I am right that it is easier
to have a greater diversity of small than of large organisms, then the evolutionary
process in small organisms will differ somewhat from that of large ones.
Wherever we have a great array of allied sympatric species there must be
an emphasis on very accurate interspecific mating barriers which is unnecessary
where virtually no sympatric allies occur. We ourselves are large animals
in this sense; it would seem very unlikely that the peculiar lability that
seems to exist in man, in which even the direction of normal sexual behavior
must be learnt, could have developed to quite the existing extent if species
recognition, involving closely related sympatric congeners, had been necessary.
Elsewhere (Hutchinson, l959) I have attempted to show that the difficulties
that Homo sapiens has to face in this regard may imply various unsuspected
processes in human evolutionary selection. But perhaps Santa Rosalia would
find at this point that we are speculating too freely, so for the moment,
while under her patronage, I will say no more.
ACKNOWLEDGMENTS
Dr. A. Minganti of the University of Palermo enabled me to collect on
Monte Pellegrino. Professor B. M. Knos of the Department of Classics of
Yale University gave me a rare and elegant word from the Greek to express
the blurring of a food chain. Dr. L. B. Slobodkin of the University of Michigan
and Dr. R. H. MacArthur of the University of Pennsylvania provided me with
their customary kinds of intellectual stimulation. To all these friends
I am most grateful.
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