DARWINISM AND THE ATOMISTIC INTERPRETATION OF INHERITANCE
SUMMARY
The failure to recognize that biology no less than physics is
an ethically neutral science is a heritage of the evolutionary
controversy. The doctrine of organic evolution evoked intense religious
hostility in the middle of the nineteenth century. Biologists were
compelled to fight for their right to speculate on their own lines.
Forced into the forum as a propagandist the biologist gave less
attention to the logical structure of the new theory than to its
apparent implications for social philosophy. The ethical concept of
progress became entangled in the evolutionary idea. In the writings of
Herbert Spencer and the evolutionist philosophers Darwinism has left a
lasting impress upon contemporary thought. Experimental biology in this
generation has undertaken the task of reducing the problems of organic
evolution to an exact science. This must necessitate a re-examination
of many traditional biological concepts and many philosophical and
sociological inferences which have been extracted from an earlier phase
in the development of the evolutionary doctrine.
V. THE METHODOLOGY OF EVOLUTION
“Chemistry is not so far from physics as the generation before ours
thought. Biology, through bio-physics and bio-chemistry, no longer
stands aloof from the methods and procedures of physical science.
And these new alliances cannot be made without modifications in
the logical construction of the separate concepts upon which these
various sciences previously took their stands. This is a task which
laboratory practice alone cannot undertake.”--_Dorothy Wrinch_
§1
From Aristotle to our own time biologists have been too preoccupied
with collecting information about the extremely complex phenomena which
they study to pay very much attention to the logical structure of the
hypotheses they adopt. This not only tends to make controversy between
the mechanist and vitalist barren, but also explains why much that has
recently been written and said about evolution is both unsatisfactory
and perplexing to the intelligent layman. Many of the views which
gained well-nigh universal assent among biologists in the latter half
of the nineteenth century have been undermined by the discoveries of
the Mendelian renaissance. When the onlooker asks the biologist for a
straightforward exposition of the present status of the evolutionary
hypothesis, he is frequently met with the guarded statement that
biologists are no longer so sure that they know _how_ evolution
occurred, but are more certain than ever that it _has_ occurred. Such
a statement might conceivably have a logically admissible meaning,
though if so, it belongs to the category of things which were better
said otherwise. On the face of it, the layman has very good reason for
wondering whether it means anything at all. It is logically permissible
to say we know that common salt is soluble, but we do not know how it
happens that common salt should possess this property. But evolution is
not a simple property. It is a process. We cannot very well know of the
existence of a process unless we can say in what the process consists.
The doctrine of evolution which deals with the way in which living
matter has come to exist in the manifold forms which biologists call
species is one which can only be placed on the same footing as the
great generalizations of physics and chemistry, when it is examined
from the experimental standpoint. From that standpoint the particular
phase in the growth of the evolutionary hypothesis associated with the
names of Darwin and Wallace has less significance than is customarily
attached to it. From a purely experimental point of view Darwin and
Wallace brought to bear on the discussion of the evolutionary doctrine
nothing which their predecessors Buffon, Erasmus Darwin, Lamarck, St.
Hilaire, Goethe and Oken lacked. The importance of their work lies in
the history of the controversy. Under Cuvier’s influence biology had
turned away from premature speculation to industrious study of the
nature of species differences from every available standpoint. Darwin
and Wallace brought together the fruits of the progress resulting
from a generation of intensive research on such lines, and formulated
the evolutionary problem in a much more explicit form than _les
philosophes_ were in a position to do. The particular answer that they
gave to the problem they formulated is the least significant part of
the contribution which Darwin and Wallace made to biological science.
The biological world did not begin to examine the experimental
implications of the selectionist solution until the rediscovery of
Mendel’s laws by Correns, de Vries and Tschermak, and their extension
to animals by Bateson and Cuenot in the opening years of the present
century. The Mendelian renaissance provoked considerable hostility from
a generation of biologists untrained in experimental methods. It is
only now becoming possible to re-examine the selectionist doctrine with
detachment and candour.
It must not be implied that antagonism to the new movement was a mere
disinclination to face the effort of learning new methods of attacking
the problem. In the nineteenth century biologists had to fight for
their right to speculate freely in their own field. The generation in
whose memory the struggles of that period were fresh not unnaturally
resented the suggestion that biologists were no longer unanimous among
themselves. It was heresy to betray the policy of a united front. If
such schisms were permitted, and the truth were allowed to leak out
to the general public, the church somnolent might again become the
church militant. The recently published biography of the late William
Bateson shows how keenly this was felt. In the end hostility towards
the new movement which followed the rediscovery of Mendel’s work gave
place to a comfortable compromise, based on the attractive device of
inventing a word for human ignorance. This device is not peculiar
to biological science. There were from the start physicists who
entertained the most profound suspicion of the ether on that account.
It was agreed to state that inheritance in animals and plants is of
two kinds, Mendelian and non-Mendelian. Study of the former was to be
encouraged because it was useful to stock breeders, horticulturalists,
and pigeon fanciers. The latter was the peculiar speciality of the
evolutionist. Apart from that, the impenitent selectionist did not
attempt to define exactly what non-Mendelian inheritance was. Its
sphere was progressively encroached upon by the Mendelian variety,
until nothing was left of it but a comfortable corner for those highly
variable characters which were somewhat vaguely referred to under the
term “quantitative inheritance,” i.e. hereditary differences in size so
subject to fluctuating variability in response to external conditions
that they are only definable by reference to a statistical average for
a particular inbred stock. Naturally experiment first turned to the
analysis of clear-cut hereditary differences such as colour, where
little trouble is requisite in standardizing external conditions, so
that an hereditary difference will be apparent in the individual. Since
mathematical analysis has been brought to bear on the study of size
inheritance in such work as that of East and Jones, there can no longer
be any justification for doubting that the atomistic conception of
heredity which Mendel formulated covers the whole domain of biparental
inheritance.
While experimental analysis was progressing towards a recognition of
the universal validity of the Mendelian conception, the brilliant
work of Morgan’s school was leading to an exact theory of the
inter-relation of genetical factors based on the observed behaviour
of the chromosomes. Experiment now equipped with a definite criterion
of genetic purity could assert that new forms do come into existence
discontinuously in nature. It could state the conditions which
determine whether a new genetic character will persist. When chromosome
maps of several allied species of the fruit-fly were constructed by
Metz and Sturtevant seven years ago the whole discussion of the problem
of species formation entered on an entirely new phase. To-day we must
approach the discussion of evolution on the assumption that in Mendel’s
atomistic conception of the hereditary process must be sought the
correct interpretation of how new characters, having come into being,
may be transmitted to future generations.
§2
To appreciate at once the greatness and the limitations of Darwin’s
contribution to evolutionary thought it is essential to see the
question in its historical perspective. Many of the steps which have
led to the construction of the evolutionary hypothesis are now only
of historical interest. Only with an understanding of the history
of the doctrine is it possible to gain a clear idea of the logical
status it occupies in scientific thought. In approaching it, one has
to remember that the discussion of organic evolution aroused a good
deal of prejudice from religious quarters, and that in consequence
many issues, e.g. Recapitulation, which were not strictly relevant to
a straightforward presentation of the problem occupied a prominent
place in the controversies that raged around it. In forming an estimate
of the present status of the evolutionary hypothesis, let us, as far
as possible, eliminate these irrelevant questions, and deal only with
the steps which have made a definite constructive contribution to the
present state of knowledge. These may be treated under four headings:
(_a_) The Principle of Biogenesis, (_b_) The Principle of Unity of
Type, (_c_) The Principle of Succession, and (_d_) The Principle of
Genetic Variation.
_The Principle of Biogenesis_ is simply the recognition that animals
and plants only arise in our immediate experience from other animals
and plants through the process of reproduction. Linnæus accepted
it in his doctrine of the fixity of species as generally true with
regard to animals in the ordinary sense. Not until the middle of the
nineteenth century did the work of Pasteur demonstrate its validity
for micro-organisms. Linnæus and Ray were among the first to recognize
the general truth of the commonplace that “like begets like.” The
Aristotelian influence which predominated during the Renaissance
had lingered on until the beginning of the seventeenth century. The
fascinating legend of the goose barnacle contained in the concluding
passage of Gerrard’s _Herbal_ (1594) is illustrated by an actual
woodcut of the Goose and its Barnacle Progenitor. The passage reads:
“But what our eyes have seene; and hands have touched we shall
declare. There is a small Island in Lancashire called the Pile of
Foulders, wherein are found the broken pieces of old and bruised
ships, some whereof have beene cast thither by shipwracke, and also
the trunks and bodies with the branches of old and rotten trees,
cast up there likewise, whereon is found a certain spume or froth
that in time breedith unto certain shells, in shape like those of
the Muskle, but sharper pointed, and of whitish colour, wherein is
contained a thing in forme like lace of silke finely woven as it
were together, of a whitish colour, one end whereof is fastened
unto the inside of the shell, even as the fish of Oisters and
Muskels are; the other end is made fast unto the belly of a rude
masse or lumpe which in time commeth to the shape of a Bird; when
it is perfectly formed the shell gapeth open, and the first thing
that appeareth is the foresaid lace or string; next come the legs
of the bird hanging out, and as it groweth greater it openeth the
shell by degrees, til at length it is all come forth and hangeth
onely by the bill: in short space after it cometh to full maturitie
and falleth unto the sea, where it gathereth feathers, and groweth
to a fowle bigger than a Mallard and less than a goose having
blacke legs and bill or beak, and feathers blacke and white,
spotted in such manner as is our magpie.... For the truth thereof
if any doubt, may it please them to repaire unto me, and I shall
satisfie them by the testimonie of good witnesses.... The bordes
and rotten planks whereon are found these shells breeding the
Barnacle are taken up on a small Island adjoyning Lancashier, halfe
a mile from the main land, called the Pile of Foulders. They spawn
as it were in March and April; the Geese are formed in May and
June, and come to fulnesse of feathers in the month after. And thus
having through God’s assistance discoursed somewhat at large of
Grasses, herbs, Shrubs, trees and Mosses, and certain Excrescences
of the earth, with other things moe, incident to the historie
thereof, we conclude and end our present Volume, with this Wonder
of England. For the which God’s Name be ever honoured and praised.”
The legend of the goose and the barnacle died a slow death, and many
diverting citations might be added. That canny Scot, Sir Robert Moray,
wrote concerning the mystery surrounding the reproductive habits of
geese and barnacles so late as 1678 in the following words, which occur
in a paper actually published in the _Royal Society’s Transactions_.
After describing the barnacle shells washed up on the coast of
Scotland, he refers to their “little bill like that of a goose, the
eyes marked, the head, neck, breast, wings, tail and feet formed, the
feathers everywhere perfectly shaped and blackish coloured, and the
feet like those of other water fowl to my best remembrance.”
Writing in the middle of the seventeenth century Sir Thomas Browne
states (_Vulgar Errors_, bk. 3):
“Concerning the generation of frogs we shall briefly deliver that
account which observation hath taught us. By frogs I understand
not such, as arising from putrefaction are bred without copulation
and because they subsist not long are called temporariæ (Rana
temporaria, the common frog), nor do I mean the little frog of an
excellent parrot green that usually sits on trees and bushes, and
is therefore called Rananculus viridis (the tree frog) but hereby I
understand the aquatile or water frog, whereof, we may behold many
millions every spring in England.”
Referring to the doubt expressed by the author of _Vulgar Errors_
concerning Aristotle’s belief that mice arise from putrefaction,
Alexander Ross commented:
“So may one doubt whether in cheese and timber worms are generated;
or if beetles and wasps in cow’s dung; or if butterflies, locusts,
grasshoppers, shell fish, snails, eels and such like be procreated
of putrefied matter which is apt to receive the form of that
creature to which it is by formative powers disposed. To question
this is to question reason, sense, and experience. If he doubt
of this let him go to Egypt, and there he will find the fields
swarming with mice, begot of the mud of Nylus, to the great
calamity of the inhabitants.”
During the sixteenth century under the influence of Vesalius, Fallopius
and Servetus experimental investigation liberated medicine from the
paralysing tradition of Galenic teleology. The effect of this change
of outlook became evident in the revival of natural history in the
seventeenth century. Redi (1688) turns to experiment to decide whether
maggots can be produced from putrescent meat, if flies are prevented
from depositing their eggs on it. “Reason, sense and experience” were
at length forced to capitulate to experiment. The comparative study of
animal life after centuries of stagnation following the publication
of Aristotle’s Natural History entered on a new phase. So long as
innumerable _ad hoc_ accounts of the origin of species existed the
general problem with which the evolutionary hypothesis deals could not
be envisaged. Thus the work of Linnæus is the starting-point of the
modern theory of evolution.
More than a century elapsed before the essential features common to
sexual reproduction in all animals were understood. Leeuwenhoek,
a Hollander, in 1668 had first seen the minute spermatozoa in the
seminal fluid. A little over a century later the ingenious Abbot
Spallanzani gave experimental proof that it is to the spermatozoan
that the seminal fluid owes its fertilizing power. Only in 1879 did
Hertwig and Fol independently observe beneath the microscope that
only one sperm normally fertilizes one egg. Their observations were
made on sea-urchins, but we now know that their conclusions are true
for all animals. Thus the recognition that everything implied in the
term inheritance has reference to the material substance of the egg
and sperm, a concept fundamental to any exact theory of hereditary
transmission, did not emerge with clarity till more than fifteen years
after the _Origin of Species_ was published.
The formal classification of organisms codified by Linnæus introduced
a new era of intensive investigation into the character of species
differences and so ushered in the great age of comparative anatomy.
Thus we come to the second step in the historical development of the
Evolution theory, the _Principle of Unity of Type_. This generalization
was the special contribution of the school of French and German
comparative anatomists whose foremost exponent was Georges Cuvier. The
work of Linnæus gave a great impetus to the study of the structural
differences between animals, at a time when anatomy like any young
branch of knowledge was still dominated by teleology. Some instructive
examples of the happy combination of piety and anatomy are given in the
_Speculum Mundi_ published by John Swan in 1635. In an old translation
of Pliny the Elder there occurs the following information about the
elephant:
“Their skin is covered with haire or bristle, no, not so much as in
their taile, which might serve them in good steade to drive away
the busie and troublesome flies (for as vast and huge a beast as he
is, the flie haunteth and stingeth him), but full their skin is of
crosse wrinckles lattiswise; and besides that, the smell thereof
is able to draw and allure such vermine to it, and therefore when
they are laid stretched along, and perceive the flies by whole
swarmes settled on their skin, sodainly they draw those cranies
and crevices together close, and so crush them all to death. This
serves them instead of taile, maine and long hairs....”
This citation is not an isolated instance of the way in which a
pagan philosopher could employ the study of natural history to
justify the ways of God to men. During the Middle Ages the influence
of ecclesiasticism reinforced the teleological attitude from which
Aristotle’s Natural History is comparatively speaking free. At a
later date Deism had its scientific complement in a tradition which
identified the pursuit of Natural History with Natural Religion. The
first classifications were based on comparatively superficial points
of resemblance. As the study of animal structure progressed in the two
generations that followed the labours of Ray and Linnæus, it became
increasingly evident that the teleological standpoint in comparative
anatomy is inadequate. If animals had been specially designed to suit
their conditions of life, it would be expected that the greatest degree
of similarity would be found in animals pursuing a similar mode of
existence. This is not what is actually found. On the contrary, as
we make the greatest degree of similarity in structure the basis of
our attempts to classify animals, our units of classification resolve
themselves into collections of forms which show the greatest diversity
of habit, locality, diet, means of progression or anything else
which might be significant from a purposive standpoint. Animals can
be classified in groups based on striking similarity in architecture
and development involving complex constellations of physiological
units. Within these groups the utmost variety of habitat, climate,
locomotion, nutrition, etc., are encountered. The underlying similarity
of the bones of the limb and its musculature in a whale, a bird and an
elephant, as contrasted with the limb structures of a beetle, a fish
or a squid illustrate this conclusion. The whole study of systematic
zoology bears witness to it. Van Baer extended the principle of Unity
of Type to embryonic forms in 1834.
The importance of the principle of Unity of Type to the Evolutionary
hypothesis lies in the attitude which it promoted. By discouraging the
teleological approach to the diversity of animal life, it paved the
way for a naturalistic investigation of the problem. The net result
of the intensive study of comparative anatomy which progressed under
the influence of Cuvier in France and Johannes Müller in Germany was
also to show that the task of classifying animals in hard and fast
categories is at all turns embarrassed by the existence of anomalous
intermediate forms like the duck-billed platypus or the worm-like
arthropod Peripatus. Thus biological thought was becoming more and more
sympathetic towards the existence of a process of species modification.
This tendency became more sharply defined as biology took the third
great step in the development of the modern theory of evolution.
This step has been called the _Principle of Succession_. When the Law
of Unity of Type first obtained recognition, many fossils were known,
but geologists had not arrived at a general agreement concerning
the order in which the various strata had been deposited nor the
magnitude of the time which their formation occupied. By the middle of
the nineteenth century the modern doctrine (“Uniformitarianism”) had
gained assent. It now became apparent from studying the distribution
of animals in space and time, that divergent forms which exist on
the earth’s surface to-day were preceded by widely distributed forms
of a more generalized type in the past. The further we go back in
the history of any group of animals, the less do we find the same
pronounced differences as are displayed by existing members of the same
assemblage. The differentiation of species is inferred from the record
of the rocks to have been a continuous process in space and time. This
doctrine in its modern form was explicitly put forward in 1855 by
Wallace.
The masterly way in which Darwin marshalled the facts at his disposal
in presenting this aspect of the case constitutes his chief claim to
have made an enduring contribution to the Doctrine of descent. From
ancient times, but more especially from the end of the seventeenth
century onwards, the hard remains of animals were discovered and
described. Shells of molluscs which only live in water were found far
inland remote from lake, river or sea. Such relics were attributed
by the current mythology of Christian countries to the deluge that
overwhelmed the contemporaries of the Noah family. Sceptics like
Voltaire, who ventured to offer more naturalistic hypotheses, were
not more felicitous in their speculations. An exception must be made
in favour of Xenophanes (B.C. _circa_ 500) and the Arab physician
Avicenna, who, it appears, recognized fossils as remains of animals
formerly alive, and saw in them evidence of the existence of oceans
where there is now only land. A giant fossil salamander which occurs
abundantly in the Upper Miocene of Switzerland, closely related to
the Japanese salamander _Cryptobranchus japonicus_, was unearthed by
Scheuchzer in 1726, and named _Homo diluvii testis_. The motto attached
to the figure reads:
Betrübtes Beingerust von einem alten Sünder
Erweiche Herz und Sinn der neuen Bösheitskinder.
This has been translated:
Oh sad remains of bone, frame of poor Man of Sin,
Soften the heart and mind of recent sinful kin.
After the Renaissance it seems that priority in the recognition of
fossils as remains of what were once living animals is due to Steno
(1699), a Danish anatomist who taught at Padua. More than a century
later, Cuvier’s monograph on fossil remains initiated the epoch
of systematic palæontology. The effect of the researches which it
initiated was not felt till the Uniformitarian doctrine, i.e. the view
that successive strata have been deposited by a continuous process,
was generally accepted, mainly through the work of Lyell (1830).
The impiety of this new geology promoted violent controversy. In
the minutes of a meeting of the Geological Society of Great Britain
in 1840, we are told that the retiring president, Dr. Buckland,
“with a look and tone of triumph pronounced upon his opponents who
dared to question the orthodoxy of the scratches and grooves of the
glacial mountains the pains of eternal itch without the privilege
of scratching” (_Hist. Geol. Soc. Lond._, p. 142). By the middle of
the nineteenth century geologists were universally convinced that
the various strata of which the earth’s crust is composed have been
laid down in orderly succession during periods of time compared with
which that occupied by the history of human society is of negligible
duration. Once this conclusion was accepted, the study of fossils
received a new impetus and progressed rapidly under the leadership of
men like Owen, Cope and his contemporaries. Students of fossils now
began to compare the characteristics of animals in different geological
epochs, and to elucidate evidences of a continuous succession of new
forms of life transmitted to posterity in the record of the rocks. Out
of their studies the principle of succession took shape.
The geological succession of animal and plant life is demonstrated by
two features of the record. Many of the more highly specialized and
successful groups of the present day are not found to have existed at
earlier periods of the earth’s history, and were preceded by forms
which are intermediate between them and representatives of surviving
groups that were already existent before them. It is also found that
the earliest members of the great groups are usually found to be of
a more generalized type of structure than existing types. Adequate
material for drawing these conclusions is provided only by forms which
have resistant structures, such as the vertebrates, shellfish and
vascular plants.
Before we can fully appreciate the continuity of the geological record,
we have to take into account the fact that the same animals are not
found in all the different parts of the globe. One group of animals
may be confined, like the kangaroos, to Australia; one group, like the
monotypic order, in which the ant-bear is placed, to South Africa. If,
then, we know that there existed in, let us say, the Chalk Age, a small
mammal which was of a type so generalized as to form a link between the
kangaroo and the ant bear, it is most important to know whether the
barriers of ocean that now separate Australia and South Africa were as
impassable in those times as they are now; or whether this architypal
mammal lived in a situation from which it could have access to both of
these promising lands of settlement for its family. We are thus led to
ask if the process of geological succession was a continuous one both
in time and space.
To answer this question demanded a comprehensive survey of the existing
distribution of animal life on the earth, perhaps the most significant
contribution that Darwin and Wallace made to the evolutionary doctrine.
In their writings the facts of geographical distribution, facts which
were very largely based on their own first-hand observations, and not
like their erroneous views upon heredity collected from the testimony
of other persons, first received critical examination. They were forced
to conclude that no amount of ingenuity could successfully interpret
the geographical distribution of animals on a purely teleological
basis. The habitat of different kinds of animals is not uniquely
determined by their special suitability for the locality in which they
occur. This statement is attested by many species that were at one
time restricted to a very definite area. When introduced into other
parts by man they have flourished phenomenally. A familiar instance
is the introduction of rabbits into Australia. The facts about the
geographical distribution of living and fossil species collected
by Darwin and Wallace resulted in an extension of the principle of
geological succession. This is especially associated with the name of
Wallace. Wallace’s law (1855) is stated briefly at the conclusion of
the memoir entitled _On the Law which has Regulated the Introduction of
New Species_. “Every species has come into existence coincident both
in space and time with a pre-existing closely allied species.”
With the statement of this law and its confirmation by more carefully
sifted and comprehensive data the positive contribution of the
nineteenth century to the development of the modern theory of evolution
ended. The picture of a progressive gradual differentiation of animal
life, as it spread over different parts of the earth in successive
geological epochs became a commonplace of the naturalistic outlook. It
remained for the Mendelian renaissance to clarify the conception of
this gradual differentiation as an outcome of the agency of natural
generation. It must be remembered that the Principle of Succession
is still only a step in the formulation of a theory of Evolution.
We have still to ascertain what is the natural process by which
this progressive differentiation has been effected. The principle
of Biogenesis forces us to look to the reproductive process for the
answer; but only experiment can arbitrate in this field. The weaving
together of principles derived from anatomy, embryology and geology
in the light that experiment throws on the nature of the reproductive
process is necessary to the completion of the evolutionary argument.
Let us now examine how much we have proved up to this point. We have
seen that animals only come into being in our immediate experience
through the agency of natural generation. We have also seen that
similarity which animals display in their hereditable properties
must be interpreted primarily in terms of the hereditability of the
properties themselves, and not in terms of a purposive agency. Finally
we have found evidence of a gradual and accumulative divergence in the
hereditable properties of animals continuing over vast geological
epochs. We have still to interpret this divergence in terms of the
only agency through which living matter in our experience is brought
into being. We are thus led to the fourth step in our argument, the
enunciation of the _Principle of Genetic Variation_.
This states the experimental fact that units of living matter with
new hereditable properties do actually come into being in the normal
operations of the process of natural generation. In using the word
_experimental_ in this connexion we lay bare a sharp divergence of
standpoint between Darwin’s generation and our own. Darwin collected
a good deal of information about the origin of domesticated plants
and animals. This seemed to his immediate successors to constitute
sufficient evidence for believing that new hereditable properties arise
in nature. The development of Mendelian analysis has shown that this
is far from certain. Unless we have studied the parent stock under
experimental conditions which safeguard its purity, we cannot be sure
that a new domesticated variety is anything more than a new combination
of genetical characters already present in pre-existing varieties. In
other words it may only have arisen through hybridization. We have now
at our disposal a clear concept of genetical purity and well-defined
methods for establishing the purity of a stock. The whole question has
been placed on a new foundation during the past three years by the
_artificial_ production of _mutants_ or sports by X-rays in pure stocks
of the fruit-fly Drosophila reared under experimental conditions.
A further discussion of the Principle of Genetical Variation with
special reference to the Selection doctrine will be undertaken in a
subsequent essay, when the possibility of building up new varieties
into the units which biologists call species will be dealt with
more fully. To return to the discussion of the logical status of the
evolutionary doctrine, we may assume that the Principle of Genetical
Variation is established. On this assumption we may state the
conclusion of the foregoing survey in the following terms. Animals with
new hereditable properties have appeared successively with increasing
divergence of type in the past history of the earth. Animals only arise
in our experience by reproduction from pre-existing animals. Animals
with new hereditable properties can arise in our immediate experience
by reproduction from pre-existing animals with different hereditable
properties. It is therefore natural to conclude that the existing
divergence of specific characteristics is the outcome of a natural
process of generation operating over long periods of geological time.
§3
Darwin’s generation was in the main satisfied with the evidence
derived from domestication. This was embodied as an _argumentum ad
hominem_ in the Selection hypothesis. The immediate effect of Darwin’s
influence was thus, as Punnett has remarked, “to divert interest from
the study of the origin of species” as an experimental issue. Zoology
and physiology became divorced in Great Britain. One resolved itself
into a Somerset House for the Animal Kingdom, tracing pedigrees on a
purely armorial basis. The other tended to develop in association with
narrowly clinical objectives, till the rise of modern experimental
zoology in the twentieth century. In the light of modern research the
Selection hypothesis presents some interesting methodological aspects
discussed elsewhere. Let us here confine ourselves to the evolutionary
hypothesis in broad outline.
There are two fundamental results of the present enquiry which must
be emphasized in any discussion of the logical structure of the
evolutionary doctrine. One is the necessity of distinguishing between
the Principle of Succession and the evolutionary hypothesis itself.
The other is the recognition that in the last resort the validity
of the evolutionary hypothesis rests on the issue of experiment.
The first of these may sound like a platitude. It is frequently
overlooked. Presumably when a biologist says we are more certain
than ever to-day that evolution has occurred, but less certain about
_how_ it has occurred, he really means that the enormous extension
of our knowledge of fossils has placed the Principle of Succession
on a much firmer foundation than it enjoyed in Darwin’s time. The
mass of new information about comparative anatomy now available is
more than ever inexplicable on a crudely teleological basis and more
than ever consistent with an evolutionary interpretation, if such an
interpretation is permissible. But evolution is more than succession.
It is the interpretation of succession in terms of genetical variation.
If experiment does not justify this interpretation, in other words if
we do not know _how_ evolution occurred, it is evident that we cannot
be more certain that it has occurred.
The critique of evolution is not exhausted by a logical analysis of
the experimental postulates of the hypothesis, because the doctrine
of succession is more than a question of fact. It also implies the
validity of current geological doctrines, whose logical status
lies outside our present enquiry.[5] What are ordinarily called
scientific hypotheses may be classified in two categories according
to the test of validity which is applied to them. They might be
called respectively _prospective_ and _interpretative_ for lack of
existing terminology which makes the distinction which is relevant
to our present object. If the consequences of one or other of a set
of hypotheses each capable of accounting for a given series of data
are uniquely capable of yielding verifiable conclusions about other
realms of our experience, we accept the hypothesis which leads us
to the new and previously undiscovered fact. We do this even if, in
the absence of the new fact, the hypothesis so verified is a _less
economical_ one than others which satisfied the original data but
do not account for the new one. By _prospective_ hypotheses I mean
hypotheses to which this test is applicable. They are such as permit
us to make verifiable predictions in other fields of experience. In
everyday language they assist us to prophesy correctly about future
events. They constitute a hierarchy of socialized beliefs. By their
aid mankind has been permitted to construct modern civilization. They
possess to a pre-eminent degree the quality of _publicity_ defined in
an earlier essay. Mendel’s hypothesis and the kinetic theory of gases
belong to this category. Some writers, among others William James, have
tended to imply that all so-called scientific hypotheses are of this
type. This is not so. There are hypotheses whose justification resides
only in the fact that they conform to the requirements of economy of
thought. Such hypotheses are accepted because alternative hypotheses
are less economical. They are incapable of yielding any verifiable
consequences which follow uniquely from them. It is these to which I
refer by the term _interpretative_ hypotheses. We construct them, not
because they are practically serviceable to us, but because they are
conformable with the intellectual requirements of a civilization which
is the practical outcome of the application of science. They share
two pre-eminent characteristics of the prospective type, economy of
hypothesis and ethical neutrality. The need for them resides in our
curiosity. They represent one aspect of the secularization of human
life and the obsolescence of animistic ideas. We construct them for
their _philosophic_ interest alone. The evolutionary doctrine belongs
to this category.
Few biologists would admit so heretical a conclusion. They would
argue that every new missing link whose discovery is almost daily
announced in the press provides verification of the predictions of the
evolutionary hypothesis. But there is a fallacy in this contention.
The discovery of missing links is not a unique consequence of the
evolutionary doctrine. It might be inferred from the Principle of
Succession, even if the evolutionary interpretation of the Principle
of Succession turned out to be incorrect. Given the experimental
postulates of the evolutionary hypothesis as established facts, the
evolutionary hypothesis does not belong to the same hierarchy of
scientific generalizations as the kinetic theory of gases or Mendel’s
Law, because as yet we are not able to predict with the aid of it any
unique consequences which can be made the issue of decisive tests.
There is an interesting consequence of these considerations, and one
which has a more comprehensive significance. Biology deals with two
kinds of relations: relations between living and non-living matter and
relations between different kinds of living matter. The Mechanistic
Conception of Life is a secular extension of experimental analysis of
the former, just as the evolutionary hypothesis is a secular extension
of experimental study of the latter. Both belong to the category of
interpretative hypotheses in the sense defined above. Why is it then
that so many prefer the luxury of scepticism concerning the first
issue, and resent the exercise of a suspicion of scepticism concerning
the second? Perhaps the answer is that evolution has already become
incorporated in the apparatus of what Robert Briffault calls custom
thought. I do not think that the physiologist who adopts the attitude
of Gallio towards the mechanistic conception of life, affecting to
despise all mere philosophy, is consistent, unless he is prepared to
dismiss the doctrine of Organic Descent in the same manner. I have yet
to meet one who does. Evolution is a philosophy.
VI. THE PROBLEM OF SPECIES
“The effect of Darwin’s _Origin of Species_ was to divert attention
from the way in which species originate.”--R. C. Punnett,
_Mendelism_
§1
In a letter to H. de Varigny dated November 25, 1891, Thomas Henry
Huxley wrote: “I shall be very glad to have your book on Experimental
Evolution. I insisted on the necessity of obtaining experimental proof
of the possibility of obtaining virtually infertile breeds from a
common stock in 1860.... From the first I told Darwin this was the
weak point of his case from the point of view of scientific logic.
But in this matter we are just where we were thirty years ago.” In
this passage Huxley explicitly draws attention to the fact that
Darwin never came to grips with the historic problem of the Origin of
Species, as it had been propounded by Linnæus. Three years later he
is writing to acknowledge the receipt of Bateson’s _Materials for the
Study of Variation_, a book which laid the philosophical foundations
of the present era of experimental enquiry into evolutionary problems.
“I see,” he notes, “you are inclined to advocate the possibility of
considerable _saltus_ on the part of Dame Nature in her variations.
I always took the same view, much to Darwin’s disgust, and we used
often to debate it.” Another thirty years passed by, and Bateson
ventured to appeal to his contemporaries for a reconsideration of the
traditional species problem in the light of the accumulated results
of investigation based on Mendel’s methods. He was rebuffed by a
veritable storm of criticism from Huxley’s followers. Evolution had
become Darwin, as geometry has become Euclid. Had Huxley been living,
it hardly seems likely that he would have taken the same side as his
devoted disciples in the controversy which ensued.
During the latter half of the eighteenth and the beginning of the
nineteenth century biological science progressed towards a clear
definition of the problem of Man’s secular origin. This progress
involved the rejection of many of the teleological concepts which had
been current since the Middle Ages. In the light of recent advances
in the study of inheritance and variation, we know that much of the
evidence which seemed adequate for an understanding of the evolutionary
process fifty years ago must be re-examined to-day and supplemented
from other sources. The final court of appeal in the case for an
evolutionary interpretation of the origin of species is experiment.
Only experiment can place the Principle of Genetic Variation, i.e.
the origin of new genetic types in the normal course of procreation,
on a sure foundation. A detailed examination of the evidence for this
conclusion is essential to a satisfactory examination of the logical
status of evolution in the light of modern knowledge. Four separate
issues suggest themselves for discussion in a critical enquiry into
the experimental evidence for the Principle of Genetic Variation.
We must first ask whether the origin of new hereditable types under
experimentally controlled conditions is an established fact. We must
then decide what natural agency ensures that new types having so arisen
will be preserved. This leads us to ask if the appearance of new types
is an occurrence of sufficient frequency to have accounted for all the
divergency of specific form that has come about in the interval of
time which geology places at our disposal. Finally we are faced with
the task of deciding how new genetic types can be segregated into the
units which biologists call species.
First let us consider the origin of new hereditable types. Thirty years
of controlled experiment on the lines suggested by Mendel’s work has
given abundant proof that from time to time there do arise in pure
stocks individuals which have entirely new hereditable properties. Such
individuals are called _mutants_ or sports, a term used synonymously
by some writers with the alternative word mutations. The word mutation
was originally employed by De Vries in a somewhat different sense from
that in which the term mutant is now used. It is preferable to avoid
perpetuating this confusion.[6] A new phase in this aspect of the
evolutionary problem has been initiated by the recent work of Müller. A
controllable agency, exposure of parents to X-rays, has been shown to
produce mutants in the fruit-fly Drosophila.
Darwin and Wallace are usually given the credit of first emphasizing
the fact of genetical variation. A careful study of their works shows
that they did not clearly apprehend the essential aspect of the
problem or realize the imperative necessity of subjecting the issue to
direct experimental test. When they spoke of variation they included
both genetical variation, i.e. the production of mutants as defined
above, and differences between parents and offspring which result from
the influence of external agencies in early development. The small
differences of which Darwin was thinking were mainly of bodily rather
than germinal origin. As such they have nothing to do with the problem
of evolution unless, as Darwin himself did, we accept the Lamarckian
doctrine. In the Introduction to the _Origin of Species_ Darwin states
his position thus: “Any being, if it vary in any manner profitable to
itself, under the complex and sometimes varying conditions on life,
will have a better chance of surviving, and thus be naturally selected.
From the strong principle of inheritance, any selected variety will
tend to propagate its new and modified form.” What he meant by the
strong principle of inheritance Darwin never states in exact terms.
Experimental knowledge was not ripe. Biology was still in the phase of
_a priori_ reasoning from “common-sense” principles. That he did not
distinguish between bodily and germinal differences is shown by the
following passage from Chapter 3 of the _Origin of Species_:
“_Variations, however slight, and from whatever cause_ proceeding,
if they be in any degree profitable to the individuals of a
species, in their infinitely complex relations to the individuals
of a species... will tend to the preservation of such individuals
and will generally be inherited by the offspring. The offspring
also will have a better chance of surviving, for of the many
individuals of a species which are periodically born, but a small
number can survive. _I have called this principle, by which each
slight variation if useful is preserved, by the term Natural
Selection._”--(Italics inserted.)
§2
The second aspect of the problem of genetical variation, formulated
above, is the special issue raised by the selection hypothesis of
Darwin and Wallace. Mendel might perhaps more justly be given priority
for clearly envisaging the essence of the problem.
“Those,” wrote Mendel, “who survey the work done in this department
will arrive at the conviction that among all the numerous
experiments made not one has been carried out to such an extent
and in such a way as to make it possible to determine the number
of different forms under which the offspring of hybrids appear, or
to arrange these forms with certainty according to their separate
generations or definitely to ascertain their statistical relations.
It requires indeed some courage to undertake a labour of such
far-reaching extent. This appears, however, to be _the only right
way by which we can finally_ reach the solution of a question the
importance of which cannot be overestimated in connexion with the
history of _the evolution of organic forms_.”
Mendel’s method shows us that so long as they attain sexual maturity
and bear offspring, new forms having once arisen, transmit their
hereditable properties unchanged. The new hereditary type will sooner
or later appear among subsequent generations in its original purity.
This prompts us to ask what chance a given mutant has of surviving to
sexual maturity. The question demands serious consideration. We know
that a very small percentage of animals that are born into the world
do actually survive till the age at which reproduction is possible. It
has been calculated that if all the progeny of a single female aphis
(the green plant louse) survived in every generation the total of
individuals produced in twelve generations would be 10^{22}. Since a
single aphis is about a tenth of an inch long, this number would cover
the face of the globe. Twelve generations in a family of aphids would
appear in less than three years. Evidently the chance that a given
mutant will survive depends on two things. One is whether it possesses
any characteristics which favour its survival in preference to the
parent form. The other is whether it appears once or many times in the
same stock. We now know that the same mutants appear again and again.
There seem to be definite _loci of instability_ on the chromosomes.
Which of these two considerations is of greater importance is at
present problematical. Most biologists incline with good reason to
regard the former as more significant. The significance of the second
is increasingly realized.
Many contemporary authors use the term Natural Selection to imply
that competition for the means of existence permits some mutants to
live, and weeds out others. On grounds of priority this can hardly be
regarded as justified by the writings of the Selectionist writers of
the nineteenth century. It is not supported by the actual words Darwin
used to define the term Natural Selection which he himself introduced.
Goodrich in his admirable book entitled _Living Organisms_, makes the
following statement with regard to Darwin’s position:
“It is often said that of late years Darwinism has lost ground,
and that natural selection cannot be regarded as a satisfying
explanation of, or even as an important factor in, the process of
evolution. Doubtless there is some truth in the saying, at all
events in so far as it appears that the doctrine is not what some
misguided enthusiasts may have represented it to be, that it does
not explain everything, that many problems remain unsolved. Yet
the Darwinian theory still stands unassailable as the one and only
rational scientific explanation of evolution by ‘natural’ forces
whose action can be observed, tested and measured. Nevertheless,
the critics are quite right in demanding convincing evidence for
every step in the argument. The modern developments of the study of
hereditary and variation on Mendelian lines, far from weakening the
case for natural selection, seem to have definitely disposed of the
only rival theory, the doctrine of Lamarck, founded on the supposed
‘inheritance of acquired characters.’ Fortuitous changes in the
inherited organization, in the complex of factors transmitted, are
left as the only elements of primary importance, the only stones of
which the edifice is built.”
These remarks imply that Darwin’s successors went much further than
Darwin in asserting the creative, preservative, accumulative and
continuous character of the selection process. This is true; but Darwin
himself, in the _Origin of Species_, expressly stated what he meant
by Natural Selection in two quotations which have already been given;
and neither of these agree with what Goodrich or any modern geneticist
means when he says that he believes in natural selection. Since Darwin
introduced the term he has priority in defining its meaning. If later
biologists mean something different, when they speak of natural
selection, it would avoid confusion to coin a new term. Elsewhere
Goodrich says: “What selection alone can do is to preserve variations;”
and he quotes Darwin’s words in support. Darwin meant by _preserving_
variations something different from what a modern geneticist believes.
The modern geneticist believes that _individuals_ who possess certain
advantageous characters will survive in virtue of these advantages.
Darwin and Wallace meant that _hereditary characteristics could only
survive_ if the supposed tendency to dilution of characters by crossing
were counteracted by the elimination of individuals at the other end of
the scale of variability.
Apart from what Darwin himself said on the subject we owe some
consideration to the sense in which his contemporaries understood his
argument. Since I may be accused of tilting with a lance of straw at
a windmill of my own construction, let us refer to the section on
swamping in Wallace’s _Darwinism_. “He (Darwin) had always considered
that the chief part and, latterly, the whole of the materials with
which natural selection works was afforded by individual variations
or that amount of ever-fluctuating variability which exists in all
organisms and in all their parts...” Wallace then proceeds to quote
Romanes as saying that “if a sufficient number of individuals were thus
simultaneously and similarly modified, there need no longer be any
danger of the variety becoming _swamped by inter-crossing_.” Wallace
himself wrote as follows:
“I have already shown that every part of an organism in common
species does vary to a very considerable amount in a large number
of individuals and in the same locality; the only point that
remains to be discussed is whether any or most of these variations
are ‘beneficial.’ But every one of these consists either in
increase or diminution of size or power of the organ or faculty,
that varies.... If less size of body would be beneficial, then as
half the variations in size are above and half below the mean or
existing standard of the species, there would be ample beneficial
variations.”
The implication is that natural selection by cutting off the other
half--the ample non-beneficial variations--prevents the swamping of the
beneficial ones out of existence. We know to-day that the traditional
belief in the swamping effects of intercrossing is false. With its
rejection the _argumentum ad hominem_ which made the struggle for
existence an essential agency for preserving new hereditary properties
becomes unnecessary.
However much importance Darwin himself attributed to this aspect
of his theory of Natural Selection, he makes clear his attitude in
several passages. It cannot be doubted that the assent which he
received from his contemporaries was in large measure due to it.
Accepting the prevailing misconceptions about swamping, he showed how
an evolutionary process could and, as it then appeared, must operate.
Experimental evidence for the hereditability of the kind of variations
on which Darwin seems to have relied was not brought forward. We now
know that the kind of variations which Darwin regarded as the raw
materials for the selective process are not generally hereditable.
The wisdom of retaining the term Natural Selection may therefore be
questioned. In all probability there is another reason which in part
explains the popularity of Darwin’s theory as contrasted with the
neglect of Mendel’s pioneer labours. Natural selection was suggested
by the analogy of industrial conditions in the nineteenth century.
Once formulated as a universal principle of nature it appealed to
the dominant political theories of the period. The Origin of Species
became the bible of _laissez faire_. It triumphed as classical humanism
triumphed during the Middle Ages in part at least for reasons which
were primarily political. The idea that the struggle for existence is a
constructive process played a prominent part in the social theories of
the Selectionist School.
A great deal of confusion can be dispelled if we recognize that Darwin
never clearly distinguished between two distinct issues. His herculean
labours in the field of geographical distribution urged him to seek a
reason for the circumstance that different species of animals exist in
different parts of the world. The struggle for existence does explain
why some species have died out in one place while others have died
out in other places. From that point Darwin went on to generalize
about the Origin of Species, i.e. how species come into being. It is
unfortunate that, though most of his earlier and enduring contributions
to science are concerned with how certain species have ceased to
exist, the title of his work laid emphasis on the process by which
species are brought into being. It was naturally this part of his
theory which made the greatest appeal to his contemporaries. Possibly
it was not the one which was most significant to Darwin himself.
Darwin used the term Natural Selection in connexion with both problems.
With regard to the former his theory is as acceptable as ever. With
regard to the latter it has now been superseded by exact experimental
enquiry into the mechanism involved in the production and preservation
of new hereditable types. The work of Gregor Mendel is the proper
starting-point of such enquiry.
A third aspect of the Principle of Genetic Variation concerns the
adequacy of geological time. It will only be touched on briefly. When
the evolutionary theory was introduced to the biological world, it
had to encounter a difficulty that no longer presents itself as a
formidable objection. Kelvin had calculated the possible period of time
during which life can have existed from considerations derived from the
rate of cooling of the earth. The allowance which Kelvin conceded was
subject to the qualification that no factors at that time undiscovered
enter into the question significantly. Since that time the discovery of
radio activity has removed the necessity to place any such restriction
on the period of geological time, as Kelvin was led to deduce. To-day
we have no reason for believing that geological time is too short to
permit us to ascribe the faunistic changes of successive generations to
the operations of the natural process of genetical variation. At the
same time the Evolution Theory will not stand side by side with the
retrospective hypotheses of astronomy in the hierarchy of scientific
generalizations, until the frequency of genetical variation and the
conditions which determine it have been correlated with more exact
knowledge of the duration and climatic features of the intervals
corresponding to geological strata.
§3
There remains another aspect of the Principle of Genetic Variation.
This is of paramount importance in connexion with the evolutionary
hypothesis. It is the Origin of Species _sensu stricto_. A good deal
of confusion has arisen in the discussion of the species problem on
account of the equivocal usage of the word _species_. It is therefore
best to begin with a clear definition of the species problem. It is a
universal experience that any dog resembles its father and mother in
more respects than it resembles any cat or any fish; any cat resembles
its father and mother in more respects than it resembles any dog or
any fish; any fish resembles its father and mother more closely than
it resembles any cat or any dog. We may express this by saying that
cats, dogs and fish have certain specific hereditable properties. If
we examine these hereditable properties we find that a cat has more
hereditable properties in common with any dog than those which it
shares with any fish. Thus organisms can be arranged or classified
in groups expressing the extent of resemblance in their hereditable
properties. The work of Ray and Linnæus in the early half of the
eighteenth century led to the general belief that “like begets like,”
and the publication of the _Systema Naturæ_ (1757) by the latter author
marks the beginning of a century and a half of detailed anatomical
studies directed to classification of this kind. According to the
degree of resemblance of organisms with respect to their hereditary
properties they are customarily grouped in phyla, classes, orders,
families, genera and species. To illustrate the meaning of these
terms let us consider the reader of this essay. He or she is said to
belong to the species _sapiens_ of the genus _Homo_, which includes
all living races of man. The genus _Homo_ includes in addition _H.
Neanderthalensis_, the early stone-age heavy-browed first men, and is
grouped with the genera _Pithecanthropus_ and _Eoanthropus_ (the fossil
ape man of Java and Pilt Down man) in a family _Hominidæ_, within the
order _Primates_, that comprises apes, monkeys and marmosets. The order
_Primates_ is one of many orders of forms within the class _Mammalia_
that includes hairy animals that suckle their young. The _Mammalia_,
along with birds, reptiles, amphibia (frogs, toads salamanders) and
fishes, is placed in the phylum _Vertebrata_, which includes all forms
with a backbone.
The degree of similarity implied by placing two species in the same
genus, order, class, etc., is an arbitrary one defined by convenience
and general consent. The degree of similarity implied in placing two
individuals in the same species in the sense in which the term was
defined by Linnæus implies something more than convenience. Linnæus
placed within the same species all individuals which breed readily with
one another. The structural difference between two Linnæan species
of animals and plants may be negligible compared with the immense
structural differences that distinguish varieties within a single
Linnæan species, as for instance the difference between White Leghorns,
Yokohamas, Silkies, Partridge Cochins, etc., which are all members of
the species _Gallus domesticus_.
Though this definition of the species as a unit is the one sanctioned
by priority, it is insufficiently emphasized by those who discuss
evolution that the creation of new species in the daily routine of a
large museum has very little to do with the Linnæan test. Preserved
animals are sent by collectors to the taxonomist, who proceeds
to classify them in new species, varieties or genera in the vast
majority of cases without any experimental knowledge as to their
breeding habits. Hence the terms species and variety are in practice
used to a large extent interchangeably, though not deliberately. The
historic problem of the origin of species is not that of the origin of
museum species but of Linnæan species. If we can show that discrete
hereditable properties arise, as we know that they do, discontinuously
in the normal course of generation, we have all the materials we
need to interpret the origin of varieties, genera, orders, families,
classes, phyla. Whereas all these are arbitrary groups defined in terms
of similarity and difference of the hereditable anatomical properties
of animals and plants, the species, as defined by Linnæus, is a group
limited not merely by the anatomical resemblance of its individual
members but also by _their inability to breed successfully with other
forms_.
What has been said so far about the origin of new hereditable
properties bears directly upon the way in which new varieties arise.
New varieties will only retain their characteristics if some external
agency is employed to prevent them from hybridizing and thereby giving
rise to an indefinite number of new combinations of characters. The
Yokohama can be made to retain those characteristic differences which
distinguish it from a White Leghorn by the mechanical device of
separating the two strains with a partition of wire netting. No wire
netting is required to prevent a White Leghorn and a turkey from losing
their genetic individualities, when they are placed in propinquity to
one another as are closely allied species in Nature. There is therefore
in addition to the problem of the origin of new varieties a problem
of the origin of species incompatibility. This cannot be dismissed as
of no importance, so long as our experimental knowledge of the origin
of varieties fails to suggest in what way this incompatibility may
arise. If we can solve this new problem the evolutionary hypothesis
presents no ulterior difficulties in the way of explaining the origin
of differences which separate the larger systematic groups. The
differences employed in distinguishing genera from varieties, and
orders from genera or classes from orders are differences of degree.
Between species, superficially at least, there seems to be a difference
in kind. On this account the origin of species has always been taken to
signify the core of the evolutionary problem.
In a somewhat panegyric vein Mr. H. G. Wells replying to Hilaire Belloc
makes the following remark: “Darwin’s book upon the subject was called
_The Origin of Species_. It was a very modest and sufficient title. He
did not even go to the length of calling it the origin of genera or
orders or classes.” Surely Darwin might much more appropriately have
employed the latter. How types which are structurally different arise
may or may not be accounted for by the selection hypothesis. How types
which will not breed with one another arise within the same stock is
not relevant to it. It is true that Darwin and Wallace vaguely referred
in their writings to a natural tendency to infertility as forms become
more sharply differentiated. This does not meet the difficulties of the
case, even if it is a sound experimental doctrine. Bateson has used
the following illustration to emphasize the irrelevance of Natural
Selection to the species problem in the strict sense of the term:
“Sometimes specific difference (anatomical differences between
species) is to be seen in a character which we can believe to
be important in the struggle, but at least as often it is some
little detail that we cannot but regard as trivial which suffices
to differentiate the two species. Even when the diagnostic point
is of such a nature that we can imagine it to make a serious
difference in the economy, we are absolutely at a loss to explain
why this feature should be necessary to species A, and unnecessary
to species B, its nearest ally. The house sparrow (_Passer
domesticus_) is in general structure very like the tree sparrow
(_P. Montanus_)... They differ in small point of colour... The two
species therefore, apart from any difference that we can suppose
to be related to respective habits, are characterized by small
fixed distinctions in colour-markings, by a striking difference in
secondary sexual characters and by a difference in variability. In
all these respects we can form no surmise as to any economic reason
why the one species should be differentiated in one way and the
other in another way, and I believe it is mere self-deception which
suggests the hope that with fuller knowledge reasons of this nature
would be discovered.”
It is permissible to argue that the final justification of the
evolutionary argument will be achieved when intersterile mutants have
been shown to appear under experimental conditions. We shall then
be able to state that new types which display not only anatomical
but specific discontinuity have arisen in the ordinary course of
generation. At present it is only possible to say that we have very
good reason to believe they can do so. Bateson overemphasized the
difficulty of the species problem when he said “the production of an
indubitably sterile hybrid from completely fertile parents, which
have arisen under critical observation from a common origin... is the
event for which we wait.” Although the origin of the species barrier
does introduce a novel issue into the discussion of the evolutionary
problem, its novelty is not so fundamental as it appears to be at first
sight. Morgan remarks with justice:
“The necessity of putting the mutation theory to the test that
Bateson calls for seems to me very doubtful, for while this is one
of the possible ways in which a mutant might split off at once from
the parent type, it is by no means the only way or even, I think,
the most probable way in which species have become separated....
There is no one problem of infertility of species and no one
problem of the sterility of hybrids, but many problems, each due to
differences that have arisen in the germinal material. One or more
of these differences may affect the mechanism of fertilization or
the process of development, producing some incompatibility.”
Bateson performed a most important task in emphasizing that the problem
of species discontinuity exists. He made its solution assume more
formidable proportions than the facts merit. There is no mysterious
wholeness about the concept of the species barrier. Like other
scientific concepts it defines a class of properties. When we examine
the characteristics of species barriers, we at once see that they
constitute a very heterogeneous assemblage of hereditable properties,
many of which are recognizably similar to hereditable properties which
we know to arise as mutants in genetic experiments. An individual may
be placed in a different species from another individual because of
some merely anatomical difference in the structures associated with the
copulative act. Owing to the respective absence of neck hackles and
tail feathers in two strains known as the Barbadoes and Rumpies, the
male of the latter cannot successfully tread the female of the former,
though each is interfertile with other breeds of domestic fowls. The
origin of such differences does not constitute a problem of a different
class from the origin of other varieties. High and low fertility are
hereditable properties that can be studied as varieties within the
species group. They have arisen as mutant characters in experiment.
If there arose within a stock mutants with complementary genes for
infertility either type would be infertile with respect to the other.
They would constitute separate species in the Linnæan sense, when the
parent stock died out. In the case of the donkey and the horse, we
can go further and identify the complementary sterility factors in
the structure of the chromosomes. Difference of size and shape in the
chromosomes of the donkey and horse prevent them from pairing in the
reduction division, so that no ripe sperm is formed in the testis of
the mule. Mutants differing with respect to chromosome numbers and
sizes arising by fragmentation or fusion are known both in plants and
animals to have arisen under experimental conditions. In many plants
they have been perpetuated by self-fertilization. Plough has raised a
mutant strain of Drosophila which is more fertile _inter se_ than with
the wild stock. The genetic basis of interspecific sterility, while
worthy of much more extensive research, is now reaching a precision
which places the experimental data of evolutionary theory beyond the
plane of Malthusian speculation.
The foregoing illustrations do not exhaust the variety of biological
characteristics which separate one individual from another as a member
of a different Linnæan species. Nor do they exhaust the types which can
be brought within the realm of experimental treatment. Other cases are
discussed at length in Crew’s _Animal Genetics_. The species barrier is
not one thing but many things. In the light of modern research there is
no reason to regard the origin of species barriers as an essentially
different problem from the origin of varieties. Nevertheless the two
issues are superficially distinct. No discussion of the present status
of the evolutionary hypothesis is complete unless the distinction is
submitted to critical examination in the light of experiment.
In the opening years of the twentieth century it had become the
fashion among biologists to treat evolution as a dogma. The growth of
experimental study of inheritance and variation tends rather to make us
value it as a hypothesis suggestive of further enquiry. The difference
between the two attitudes is akin to a difference of method which
mankind has adopted throughout the ages in the pursuit of knowledge.
One method rationalized in its most rigid form in the philosophy of
Hegel is to seek for some proposition to which every one is agreed and
proceed by deduction to whatever conclusions may be reached from the
starting-point. This method has proved invaluable to politicians and
members of the legal profession in the discharge of their vocational
activities. It is essentially like that of the schoolmen who would
exhaust themselves in untiring search into the writings of the ancients
for some authoritative statement regarding the number of teeth which
the horse possesses, a statement that no one would dare to question.
The scientific method is irreconcilably opposed to the Hegelian
method. With no aspirations to good breeding it prefers to look the
gift horse in the mouth. It is just those propositions which every
one accepts that the scientist is most anxious to examine in the hard
light of experience. In attempting to envisage a natural mechanism by
which the graded differentiation of animal structure could have been
brought about, Lamarck was content to employ the generally accepted
belief in the inheritance of acquired characters without bringing
it to experimental test. Darwin, fortified with newer knowledge of
the historical succession of animals and plants as recorded in the
rocks, sought to show that evolution was a necessary consequence of
competition and the “strong principle of inheritance.” Darwin did
not undertake the task of enquiring into the nature of the “strong
principle of inheritance.” It was to him like one of Euclid’s axioms.
Mendel alone at this time saw the necessity for an experimental study
of inheritance, and pointed the way to a non-dialectical treatment of
the problem.
VII. NATURAL SELECTION AND EXPERIMENTAL RESEARCH
“Heredity as something quite incomprehensible cannot be used as an
explanation, but only as a designation for the identification of a
problem. And the same holds good of adaptability.”--Nietzsche, _The
Will to Power_
§1
To large numbers of people evolution is Darwinism, just as to our
fathers geometry was Euclid. In one of his writings Morgan has remarked
that “it is not so important to find out whether Darwin’s ideas were
as clear as our own, as to make sure that our own ideas are clear.”
This is true; but an interest in the history of scientific thought
is a blameless pursuit for its own sake; and there are ulterior
reasons which justify an historical discussion of the criticisms which
experimental discovery has brought to bear on the Selection doctrine
in its original form. During the latter half of the nineteenth century
the evolutionary hypothesis became entangled with the idea of a moral
progress of mankind. On this account some philosophers, who are not
biologists themselves, fail to recognize the ethical neutrality of
biological enquiry. It is doubtful whether the promulgation of any
scientific hypothesis has ever had so profound and, at the same time,
so immediate an effect on the attitude of educated people towards
personal responsibility and social obligations. The fate of Darwinism
is as much the concern of the layman as of academic biologists.
Nor is it easy for those who are not biologists to gain definite
enlightenment concerning the extent of the change that has taken
place. With the rise of experimental method the discussion of evolution
has become more technical owing to the accumulation of new data and on
account of the introduction of a more intricate form of logic. It is
a quantitative branch of science. There was a time when the biologist
thought it worth his while to read and to reply to Samuel Butler.
To-day there are biologists who read--and like the present writer
enjoy--the works of Mr. Bernard Shaw. They do not feel it necessary to
defend their philosophy against the arguments advanced in the preface
to _Back to Methuselah_. Popular expositions of evolution are still
written. More often than not one suspects that they are rather too
popular to answer the questions which an intelligent reader who is not
a biologist is most anxious to hear discussed.