FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
Fundamental difficulties and objections--Mr. Herbert Spencer's
factors of organic evolution--Disuse and effects of withdrawal
of natural selection--Supposed effects of disuse among wild
animals--Difficulty as to co-adaptation of parts by variation
and selection--Direct action of the environment--The American
school of evolutionists--Origin of the feet of the
ungulates--Supposed action of animal intelligence--Semper on the
direct influence of the environment--Professor Geddes's theory
of variation in plants--Objections to the theory--On the origin
of spines--Variation and selection overpower the effects of use
and disuse--Supposed action of the environment in imitating
variations--Weismann's theory of heredity--The cause of
variation--The non-heredity of acquired characters--The theory
of instinct--Concluding remarks.
Having now set forth and illustrated at some length the most important
of the applications of the development hypothesis in the explanation of
the broader and more generally interesting phenomena presented by the
organic world, we propose to discuss some of the more fundamental
problems and difficulties which have recently been adduced by eminent
naturalists. It is the more necessary to do this, because there is now a
tendency to minimise the action of natural selection in the production
of organic forms, and to set up in its place certain fundamental
principles of variation or laws of growth, which it is urged are the
real originators of the several lines of development, and of most of the
variety of form and structure in the vegetable and animal kingdoms.
These views have, moreover, been seized upon by popular writers to throw
doubt and discredit on the whole theory of evolution, and especially on
Darwin's presentation of that theory, to the bewilderment of the general
public, who are quite unable to decide how far the new views, even if
well established, tend to subvert the Darwinian theory, or whether they
are really more than subsidiary parts of it, and quite powerless without
it to produce any effect whatever.
The writers whose special views we now propose to consider are: (1) Mr.
Herbert Spencer, on modification of structures arising from modification
of functions, as set forth in his _Factors of Organic Evolution_. (2)
Dr. E.D. Cope, who advocates similar views in detail, in his work
entitled _The Origin of the Fittest_, and may be considered the head of
a school of American naturalists who minimise the agency of natural
selection. (3) Dr. Karl Semper, who has especially studied the direct
influence of the environment in the whole animal kingdom, and has set
forth his views in a volume on _The Natural Conditions of Existence as
they Affect Animal Life_. (4) Mr. Patrick Geddes, who urges that
fundamental laws of growth, and the antagonism of vegetative and
reproductive forces, account for much that has been imputed to natural
selection.
We will now endeavour to ascertain what are the more important facts and
arguments adduced by each of the above writers, and how far they offer a
substitute for the action of natural selection; having done which, a
brief account will be given of the views of Dr. Aug. Weismann, whose
theory of heredity will, if established, strike at the very root of the
arguments of the first three of the writers above referred to.
_Mr. Herbert Spencer's Factors of Organic Evolution._
Mr. Spencer, while fully recognising the importance and wide range of
the principle of natural selection, thinks that sufficient weight has
not been given to the effects of use and disuse as a factor in
evolution, or to the direct action of the environment in determining or
modifying organic structures. As examples of the former class of
actions, he adduces the decreased size of the jaws in the civilised
races of mankind, the inheritance of nervous disease produced by
overwork, the great and inherited development of the udders in cows and
goats, and the shortened legs, jaws, and snout in improved races of
pigs--the two latter examples being quoted from Mr. Darwin,--and other
cases of like nature. As examples of the latter, Mr. Darwin is again
quoted as admitting that there are many cases in which the action of
similar conditions appears to have produced corresponding changes in
different species; and we have a very elaborate discussion of the direct
action of the medium in modifying the protoplasm of simple organisms, so
as to bring about the difference between the outer surface and the inner
part that characterises the cells or other units of which they are
formed.
Now, although this essay did little more than bring together facts which
had been already adduced by Mr. Darwin or by Mr. Spencer himself, and
lay stress upon their importance, its publication in a popular review
was immediately seized upon as "an avowed and definite declaration
against some of the leading ideas on which the Mechanical Philosophy
depends," and as being "fatal to the adequacy of the Mechanical
Philosophy as any explanation of organic evolution,"[198]--an expression
of opinion which would be repudiated by every Darwinian. For, even
admitting the interpretation which Mr. Spencer puts on the facts he
adduces, they are all included in the causes which Darwin himself
recognised as having acted in bringing about the infinitude of forms in
the organic world. In the concluding chapter of the _Origin of Species_
he says: "I have now recapitulated the facts and considerations which
have thoroughly convinced me that species have been modified during a
long course of descent. This has been effected chiefly through the
natural selection of numerous successive, slight, favourable variations;
aided in an important manner by the inherited effects of the use and
disuse of parts; and in an unimportant manner--that is, in relation to
adaptive structures whether past or present, by the direct action of
external conditions, and by variations which seem to us, in our
ignorance, to arise spontaneously." This passage, summarising Darwin's
whole inquiry, and explaining his final point of view, shows how very
inaccurate may be the popular notion, as expressed by the Duke of
Argyll, of any supposed additions to the causes of change of species as
recognised by Darwin.
But, as we shall see presently, there is now much reason to believe
that the supposed inheritance of acquired modifications--that is, of the
effects of use and disuse, or of the direct influence of the
environment--is not a fact; and if so, the very foundation is taken away
from the whole class of objections on which so much stress is now laid.
It therefore becomes important to inquire whether the facts adduced by
Darwin, Spencer, and others, do really necessitate such inheritance, or
whether any other interpretation of them is possible. I believe there is
such an interpretation; and we will first consider the cases of disuse
on which Mr. Spencer lays most stress.
The cases Mr. Spencer adduces as demonstrating the effects of disuse in
diminishing the size and strength of organs are, the diminished size of
the jaws in the races of civilised men, and the diminution of the
muscles used in closing the jaws in the case of pet-dogs fed for
generations on soft food. He argues that the minute reduction in any one
generation could not possibly have been useful, and, therefore, not the
subject of natural selection; and against the theory of correlation of
the diminished jaw with increased brain in man, he urges that there are
cases of large brain development, accompanied by jaws above the average
size. Against the theory of economy of nutrition in the case of the
pet-dogs, he places the abundant food of these animals which would
render such economy needless.
But neither he nor Mr. Darwin has considered the effects of the
withdrawal of the action of natural selection in keeping up the parts in
question to their full dimensions, which, of itself, seems to me quite
adequate to produce the results observed. Recurring to the evidence,
adduced in Chapter III, of the constant variation occurring in all parts
of the organism, while selection is constantly acting on these
variations in eliminating all that fall below the best working standard,
and preserving only those that are fully up to it; and, remembering
further, that, of the whole number of the increase produced annually,
only a small percentage of the best adapted can be preserved, we shall
see that every useful organ will be kept up nearly to its higher limit
of size and efficiency. Now Mr. Galton has proved experimentally that,
when any part has thus been increased (or diminished) by selection,
there is in the offspring a strong tendency to revert to a mean or
average size, which tends to check further increase. And this mean
appears to be, not the mean of the actual existing individuals but a
lower mean, or that from which they had been recently raised by
selection.[199] He calls this the law of "Regression towards
Mediocrity," and it has been proved by experiments with vegetables and
by observations on mankind. This regression, in every generation, takes
place even when both parents have been selected for their high
development of the organ in question; but when there is no such
selection, and crosses are allowed among individuals of every grade of
development, the deterioration will be very rapid; and after a time not
only will the average size of the part be greatly reduced, but the
instances of full development will become very rare. Thus what Weismann
terms "panmixia," or free intercrossing, will co-operate with Galton's
law of "regression towards mediocrity," and the result will be that,
whenever selection ceases to act on any part or organ which has
heretofore been kept up to a maximum of size and efficiency, the organ
in question will rapidly decrease till it reaches a mean value
considerably below the mean of the progeny that has usually been
produced each year, and very greatly below the mean of that portion
which has survived annually; and this will take place by the general law
of heredity, and quite irrespective of any _use_ or _disuse_ of the part
in question. Now, no observations have been adduced by Mr. Spencer or
others, showing that the average amount of change supposed to be due to
_disuse_ is greater than that due to the law of regression towards
mediocrity; while even if it were somewhat greater, we can see many
possible contributory causes to its production. In the case of civilised
man's diminished jaw, there may well be some correlation between the jaw
and the brain, seeing that increased mental activity would lead to the
withdrawal of blood and of nervous energy from adjacent parts, and might
thus lead to diminished growth of those parts in the individual. And in
the case of pet-dogs, the selection of small or short-headed individuals
would imply the unconscious selection of those with less massive
temporal muscles, and thus lead to the concomitant reduction of those
muscles. The amount of reduction observed by Darwin in the wing-bones of
domestic ducks and poultry, and in the hind legs of tame rabbits, is
very small, and is certainly no greater than the above causes will well
account for; while so many of the external characters of all our
domestic animals have been subject to long-continued artificial
selection, and we are so ignorant of the possible correlations of
different parts, that the phenomena presented by them seem sufficiently
explained without recurrence to the assumption that any changes in the
individual, due to disuse, are inherited by the offspring.
_Supposed Effects of Disuse among Wild Animals._
It may be urged, however, that among wild animals we have many undoubted
results of disuse much more pronounced than those among domestic kinds,
results which cannot be explained by the causes already adduced. Such
are the reduced size of the wings of many birds on oceanic islands; the
abortion of the eyes in many cave animals, and in some which live
underground; and the loss of the hind limbs in whales and in some
lizards. These cases differ greatly in the amount of the reduction of
parts which has taken place, and may be due to different causes. It is
remarkable that in some of the birds of oceanic islands the reduction is
little if at all greater than in domestic birds, as in the water-hen of
Tristan d'Acunha. Now if the reduction of wing were due to the
hereditary effects of disuse, we should expect a very much greater
effect in a bird inhabiting an oceanic island than in a domestic bird,
where the disuse has been in action for an indefinitely shorter period.
In the case of many other birds, however--as some of the New Zealand
rails and the extinct dodo of Mauritius--the wings have been reduced to
a much more rudimentary condition, though it is still obvious that they
were once organs of flight; and in these cases we certainly require some
other causes than those which have reduced the wings of our domestic
fowls. One such cause may have been of the same nature as that which has
been so efficient in reducing the wings of the insects of oceanic
islands--the destruction of those which, during the occasional use of
their wings, were carried out to sea. This form of natural selection may
well have acted in the case of birds whose powers of flight were
already somewhat reduced, and to whom, there being no enemies to escape
from, their use was only a source of danger. We may thus, perhaps,
account for the fact that many of these birds retain small but useless
wings with which they never fly; for, the wings having been reduced to
this functionless condition, no power could reduce them further except
correlation of growth or economy of nutrition, causes which only rarely
come into play.
The complete loss of eyes in some cave animals may, perhaps, be
explained in a somewhat similar way. Whenever, owing to the total
darkness, they became useless, they might also become injurious, on
account of their delicacy of organisation and liability to accidents and
disease; in which case natural selection would begin to act to reduce,
and finally abort them; and this explains why, in some cases, the
rudimentary eye remains, although completely covered by a protective
outer skin. Whales, like moas and cassowaries, carry us back to a remote
past, of whose conditions we know too little for safe speculation. We
are quite ignorant of the ancestral forms of either of these groups, and
are therefore without the materials needful for determining the steps by
which the change took place, or the causes which brought it about.[200]
On a review of the various examples that have been given by Mr. Darwin
and others of organs that have been reduced or aborted, there seems too
much diversity in the results for all to be due to so direct and uniform
a cause as the individual effects of disuse accumulated by heredity. For
if that were the only or chief efficient cause, and a cause capable of
producing a decided effect during the comparatively short period of the
existence of animals in a state of domestication, we should expect to
find that, in wild species, all unused parts or organs had been reduced
to the smallest rudiments, or had wholly disappeared. Instead of this we
find various grades of reduction, indicating the probable result of
several distinct causes, sometimes acting separately, sometimes in
combination, such as those we have already pointed out.
And if we find no positive evidence of _disuse_, acting by its direct
effect on the individual, being transmitted to the offspring, still less
can we find such evidence in the case of the _use_ of organs. For here
the very fact of _use_, in a wild state, implies _utility_, and utility
is the constant subject for the action of natural selection; while among
domestic animals those parts which are exceptionally used are so used in
the service of man, and have thus become the subjects of artificial
selection. Thus "the great and inherited development of the udders in
cows and goats," quoted by Spencer from Darwin, really affords no proof
of inheritance of the increase due to use, because, from the earliest
period of the domestication of these animals, abundant milk-production
has been highly esteemed, and has thus been the subject of selection;
while there are no cases among wild animals that may not be better
explained by variation and natural selection.
_Difficulty as to Co-adaptation of Parts by Variation and Selection._
Mr. Spencer again brings forward this difficulty, as he did in his
_Principles of Biology_ twenty-five years ago, and urges that all the
adjustments of bones, muscles, blood-vessels, and nerves which would be
required during, for example, the development of the neck and fore-limbs
of the giraffe, could not have been effected by "simultaneous fortunate
spontaneous variations." But this difficulty is fully disposed of by the
facts of simultaneous variation adduced in our third chapter, and has
also been specially considered in Chapter VI, p. 127. The best answer to
this objection may, perhaps, be found in the fact that the very thing
said to be impossible by variation and natural selection has been again
and again effected by variation and artificial selection. During the
process of formation of such breeds as the greyhound or the bulldog, of
the race-horse and carthorse, of the fantail pigeon or the otter-sheep,
many co-ordinate adjustments have been produced; and no difficulty has
occurred, whether the change has been effected by a single variation--as
in the last case named--or by slow steps, as in all the others. It seems
to be forgotten that most animals have such a surplus of vitality and
strength for all the ordinary occasions of life that any slight
superiority in one part can be at once utilised; while the moment any
want of balance occurs, variations in the insufficiently developed parts
will be selected to bring back the harmony of the whole organisation.
The fact that, in all domestic animals, variations do occur, rendering
them swifter or stronger, larger or smaller, stouter or slenderer, and
that such variations can be separately selected and accumulated for
man's purposes, is sufficient to render it certain that similar or even
greater changes may be effected by natural selection, which, as Darwin
well remarks, "acts on every internal organ, on every shade of
constitututional difference, on the whole machinery of life." The
difficulty as to co-adaptation of parts by variation and natural
selection appears to me, therefore, to be a wholly imaginary difficulty
which has no place whatever in the operations of nature.
_Direct Action of the Environment._
Mr. Spencer's last objection to the wide scope given by Darwinians to
the agency of natural selection is, that organisms are acted upon by the
environment, which produces in them definite changes, and that these
changes in the individual are transmitted by inheritance, and thus
become increased in successive generations. That such changes are
produced in the individual there is ample evidence, but that they are
inherited independently of any form of selection or of reversion is
exceedingly doubtful, and Darwin nowhere expresses himself as satisfied
with the evidence. The two very strongest cases he mentions are the
twenty-nine species of American trees which all differed in a
corresponding way from their nearest European allies; and the American
maize which became changed after three generations in Europe. But in the
case of the trees the differences alleged may be partly due to
correlation with constitutional peculiarities dependent on climate,
especially as regards the deeper tint of the fading leaves and the
smaller size of the buds and seeds in America than in Europe; while the
less deeply toothed or serrated leaves in the American species are, in
our present complete ignorance of the causes and uses of serration,
quite as likely to be due to some form of adaptation as to any direct
action of the climate. Again, we are not told how many of the allied
species do not vary in this particular manner, and this is certainly an
important factor in any conclusion we may form on the question.
In the case of the maize it appears that one of the more remarkable and
highly selected American varieties was cultivated in Germany, and in
three years nearly all resemblance to the original parent was lost; and
in the sixth year it closely resembled a common European variety, but
was of somewhat more vigorous growth. In this case no selection appears
to have been practised, and the effects may have been due to that
"reversion to mediocrity" which invariably occurs, and is more
especially marked in the case of varieties which have been rapidly
produced by artificial selection. It may be considered as a partial
reversion to the wild or unimproved stock; and the same thing would
probably have occurred, though perhaps less rapidly, in America itself.
As this is stated by Darwin to be the most remarkable case known to him
"of the direct and prompt action of climate on a plant," we must
conclude that such direct effects have not been proved to be accumulated
by inheritance, independently of reversion or selection.
The remaining part of Mr. Spencer's essay is devoted to a consideration
of the hypothetical action of the environment on the lower organisms
which consist of simple cells or formless masses of protoplasm; and he
shows with great elaboration that the outer and inner parts of these
are necessarily subject to different conditions; and that the outer
actions of air or water lead to the formation of integuments, and
sometimes to other definite modifications of the surface, whence arise
permanent differences of structure. Although in these cases also it is
very difficult to determine how much is due to direct modification by
external agencies transmitted and accumulated by inheritance, and how
much to spontaneous variations accumulated by natural selection, the
probabilities in favour of the former mode of action are here greater,
because there is no differentiation of nutritive and reproductive cells
in these simple organisms; and it can be readily seen that any change
produced in the latter will almost certainly affect the next
generation.[201] We are thus carried back almost to the origin of life,
and can only vaguely speculate on what took place under conditions of
which we know so little.
_The American School of Evolutionists._
The tentative views of Mr. Spencer which we have just discussed, are
carried much further, and attempts have been made to work them out in
great detail, by many American naturalists, whose best representative is
Dr. E.D. Cope of Philadelphia.[202] This school endeavours to explain
all the chief modifications of form in the animal kingdom by fundamental
laws of growth and the inherited effects of use and effort, returning,
in fact, to the teachings of Lamarck as being at least equally important
with those of Darwin.
The following extract will serve to show the high position claimed by
this school as original discoverers, and as having made important
additions to the theory of evolution:
"Wallace and Darwin have propounded as the cause of modification in
descent their law of natural selection. This law has been epitomised by
Spencer as the 'survival of the fittest.' This neat expression no doubt
covers the case, but it leaves the origin of the fittest entirely
untouched. Darwin assumes a 'tendency to variation' in nature, and it is
plainly necessary to do this, in order that materials for the exercise
of a selection should exist. Darwin and Wallace's law is then only
restrictive, directive, conservative, or destructive of something
already created. I propose, then, to seek for the originative laws by
which these subjects are furnished; in other words, for the causes of
the origin of the fittest."[203]
Mr. Cope lays great stress on the existence of a special developmental
force termed "bathmism" or growth-force, which acts by means of
retardation and acceleration "without any reference to fitness at all;"
that "instead of being controlled by fitness it is the controller of
fitness." He argues that "all the characteristics of generalised groups
from genera up (excepting, perhaps, families) have been evolved under
the law of acceleration and retardation," combined with some
intervention of natural selection; and that specific characters, or
species, have been evolved by natural selection with some assistance
from the higher law. He, therefore, makes species and genera two
absolutely distinct things, the latter not developed out of the former;
generic characters and specific characters are, in his opinion,
fundamentally different, and have had different origins, and whole
groups of species have been simultaneously modified, so as to belong to
another genus; whence he thinks it "highly probable that the same
specific form has existed through a succession of genera, and perhaps in
different epochs of geologic time."
Useful characters, he concludes, have been produced by the special
location of growth-force by use; useless ones have been produced by
location of growth-force without the influence of use. Another element
which determines the direction of growth-force, and which precedes use,
is effort; and "it is thought that effort becomes incorporated into the
metaphysical acquisitions of the parent, and is inherited with other
metaphysical qualities by the young, which, during the period of growth,
is much more susceptible to modifying influences, and is likely to
exhibit structural change in consequence."[204]
From these few examples of their teachings, it is clear that these
American evolutionists have departed very widely from the views of Mr.
Darwin, and in place of the well-established causes and admitted laws to
which he appeals have introduced theoretical conceptions which have not
yet been tested by experiments or facts, as well as metaphysical
conceptions which are incapable of proof. And when they come to
illustrate these views by an appeal to palaeontology or morphology, we
find that a far simpler and more complete explanation of the facts is
afforded by the established principles of variation and natural
selection. The confidence with which these new ideas are enunciated, and
the repeated assertion that without them Darwinism is powerless to
explain the origin of organic forms, renders it necessary to bestow a
little more time on the explanations they give us of well-known
phenomena with which, they assert, other theories are incompetent to
grapple.
As examples of use producing structural change, Mr. Cope adduces the
hooked and toothed beaks of the falcons and the butcher-birds, and he
argues that the fact of these birds belonging to widely different groups
proves that similarity of use has produced a similar structural result.
But no attempt is made to show any direct causal connection between the
use of a bill to cut or tear flesh and the development of a tooth on the
mandible. Such use might conceivably strengthen the bill or increase its
size, but not cause a special tooth-like outgrowth which was not present
in the ancestral thrush-like forms of the butcher-bird. On the other
hand, it is clear that any variations of the bill tending towards a hook
or tooth would give the possessor some advantage in seizing and tearing
its prey, and would thus be preserved and increased by natural
selection. Again, Mr. Cope urges the effects of a supposed "law of polar
or centrifugal growth" to counteract a tendency to unsymmetrical growth,
where one side of the body is used more than the other. But the
undoubted hurtfulness of want of symmetry in many important actions or
functions would rapidly eliminate any such tendency. When, however, it
has become useful, as in the case of the single enlarged claw of many
Crustacea, it has been preserved by natural selection.
_Origin of the Feet of the Ungulates._
Perhaps the most original and suggestive of Mr. Cope's applications of
the theory of use and effort in modifying structure are, his chapters
"On the Origin of the Foot-Structure of the Ungulates;" and that "On the
Effect of Impacts and Strains on the Feet of Mammalia;" and they will
serve also to show the comparative merits of this theory and that of
natural selection in explaining a difficult case of modification,
especially as it is an explanation claimed as new and original when
first enunciated in 1881. Let us, then, see how he deals with the
problem.
The remarkable progressive change of a four or five-toed ancestor into
the one-toed horse, and the equally remarkable division of the whole
group of ungulate animals into the odd-toed and even-toed divisions, Mr.
Cope attempts to explain by the effects of impact and use among animals
which frequented hard or swampy ground respectively. On hard ground, it
is urged, the long middle toe would be most used and subjected to the
greatest strains, and would therefore acquire both strength and
development. It would then be still more exclusively used, and the extra
nourishment required by it would be drawn from the adjacent less-used
toes, which would accordingly diminish in size, till, after a long
series of changes, the records of which are so well preserved in the
American tertiary rocks, the true one-toed horse was developed. In soft
or swampy ground, on the other hand, the tendency would be to spread out
the foot so that there were two toes on each side. The two middle toes
would thus be most used and most subject to strains, and would,
therefore, increase at the expense of the lateral toes. There would be,
no doubt, an advantage in these two functional toes being of equal size,
so as to prevent twisting of the foot while walking; and variations
tending to bring this about would be advantageous, and would therefore
be preserved. Thus, by a parallel series of changes in another
direction, adapted to a distinct set of conditions, we should arrive at
the symmetrical divided hoofs of our deer and cattle. The fact that
sheep and goats are specially mountain and rock-loving animals may be
explained by their being a later modification, since the divided hoof
once formed is evidently well adapted to secure a firm footing on rugged
and precipitous ground, although it could hardly have been first
developed in such localities. Mr. Cope thus concludes: "Certain it is
that the length of the bones in the feet of the ungulate orders has a
direct relation to the dryness of the ground they inhabit, and the
possibility of speed which their habit permits them or necessarily
imposes on them."[205]
If there is any truth in the explanation here briefly summarised, it
must entirely depend on the fact of individual modifications thus
produced being hereditary, and we yet await the proof of this. In the
meantime it is clear that the very same results could have been brought
about by variation and natural selection. For the toes, like all other
organs, vary in size and proportions, and in their degree of union or
separation; and if in one group of animals it was beneficial to have the
middle toe larger and longer, and in another set to have the two middle
toes of the same size, nothing can be more certain than that these
particular modifications would be continuously preserved, and the very
results we see ultimately produced.
The oft-repeated objections that the cause of variations is unknown,
that there must be something to determine variations in the right
direction; that "natural selection includes no actively progressive
principle, but must wait for the development of variation, and then,
after securing the survival of the best, wait again for the best to
project its own variations for selection," we have already sufficiently
answered by showing that variation--in abundant or typical species--is
always present in ample amount; that it exists in all parts and organs;
that these vary, for the most part, independently, so that any required
combination of variations can be secured; and finally, that all
variation is necessarily either in excess or defect of the mean
condition, and that, consequently, the right or favourable variations
are so frequently present that the unerring power of natural selection
never wants materials to work upon.
_Supposed Action of Animal Intelligence._
The following passage briefly summarises Mr. Cope's position:
"Intelligence is a conservative principle, and will always direct effort
and use into lines which will be beneficial to its possessor. Here we
have the source of the fittest, _i.e._ addition of parts by increase and
location of growth-force, directed by the influence of various kinds of
compulsion in the lower, and intelligent option among higher animals.
Thus intelligent choice, taking advantage of the successive evolution of
physical conditions, may be regarded as the _originator of the fittest_,
while natural selection is the tribunal to which all results of
accelerated growth are submitted. This preserves or destroys them, and
determines the new points of departure on which accelerated growth shall
build."[206]
This notion of "intelligence"--the intelligence of the animal
itself--determining its own variation, is so evidently a very partial
theory, inapplicable to the whole vegetable kingdom, and almost so to
all the lower forms of animals, amongst which, nevertheless, there is
the very same adaptation and co-ordination of parts and functions as
among the highest, that it is strange to see it put forward with such
confidence as necessary for the completion of Darwin's theory. If "the
various kinds of compulsion"--by which are apparently meant the laws of
variation, growth, and reproduction, the struggle for existence, and the
actions necessary to preserve life under the conditions of the animal's
environment--are sufficient to have developed the varied forms of the
lower animals and of plants, we can see no reason why the same
"compulsion" should not have carried on the development of the higher
animals also. The action of this "intelligent option" is altogether
unproved; while the acknowledgment that natural selection is the
tribunal which either preserves or destroys the variations submitted to
it, seems quite inconsistent with the statement that intelligent choice
is the "orginator of the fittest," since whatever is really "the
fittest" can never be destroyed by natural selection, which is but
another name for the survival of the fittest. If "the fittest" is always
definitely produced by some other power, then natural selection is not
wanted. If, on the other hand, both fit and unfit are produced, and
natural selection decides between them, that is pure Darwinism, and Mr.
Cope's theories have added nothing to it.
[Illustration: FIG. 35.--Transformation of Artemia salina to A.
Milhausenii; 1, tail-lobe of A. salina, and its transition through
2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A.
salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A.
Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)]
_Semper on the Direct Influence of the Environment._
Another eminent naturalist, Professor Karl Semper of Würzburg, also
adopts the view of the direct transforming power of the environment, and
has brought together an immense body of interesting facts showing the
influence of food, of light, of temperature, of still water and moving
water, of the atmosphere and its currents, of gravitation, and of other
organisms, in modifying the forms and other characteristics of
animals.[207] He believes that these various influences produce a direct
and important effect, and that this effect is accumulated by
inheritance; yet he acknowledges that we have no direct evidence of
this, and there is hardly a single case adduced in the book which is not
equally well explained by adaptation, brought about by the survival of
beneficial variations. Perhaps the most remarkable case he has brought
forward is that of the transformation of species of crustaceans by a
change in the saltness of the water (see Fig. 35). Artemia salina lives
in brackish water, while A. Milhausenii inhabits water which is much
salter. They differ greatly in the form of the tail-lobes, and in the
presence or absence of spines upon the tail, and had always been
considered perfectly distinct species. Yet either was transformed into
the other in a few generations, during which the saltness of the water
was gradually altered. Yet more, A. salina was gradually accustomed to
fresher water, and in the course of a few generations, when the water
had become perfectly fresh, the species was changed into Branchipus
stagnalis, which had always been considered to belong to a different
genus on account of differences in the form of the antennae and of the
posterior segments of the body (see Fig. 36). This certainly appears to
be a proof of change of conditions producing a change of form
independently of selection, and of that change of form, while remaining
under the same conditions, being inherited. Yet there is this
peculiarity in the case, that there is a chemical change in the water,
and that this water permeates the whole body, and must be absorbed by
the tissues, and thus affect the ova and even the reproductive
elements, and in this way may profoundly modify the whole organisation.
Why and how the external effects are limited to special details of the
structure we do not know; but it does not seem as if any far-reaching
conclusions as to the cumulative effect of external conditions on the
higher terrestrial animals and plants, can be drawn from such an
exceptional phenomenon. It seems rather analogous to those effects of
external influences on the very lowest organisms in which the vegetative
and reproductive organs are hardly differentiated, in which case such
effects are doubtless inherited.[208]
[Illustration: FIG. 36. _a._ Branchipus stagnalis. _b._ Artemia salina.]
_Professor Geddes's Theory of Variation in Plants._
In a paper read before the Edinburgh Botanical Society in 1886 Mr.
Patrick Geddes laid down the outlines of a fundamental theory of plant
variation, which he has further extended in the article "Variation and
Selection" in the _Encydopaedia Britannica_, and in a paper read before
the Linnaean Society but not yet published.
A theory of variation should deal alike with the origin of specific
distinctions and with those vaster differences which characterise the
larger groups, and he thinks it should answer such questions as--How an
axis comes to be arrested to form a flower? how the various forms of
inflorescence were evolved? how did perigynous or epigynous flowers
arise from hypogynous flowers? and many others equally fundamental.
Natural selection acting upon numerous accidental variations will not,
he urges, account for such general facts as these, which must depend on
some constant law of variation. This law he believes to be the
well-known antagonism of vegetative and reproductive growth acting
throughout the whole course of plant development; and he uses it to
explain many of the most characteristic features of the structure of
flowers and fruits.
Commencing with the origin of the flower, which all botanists agree in
regarding as a shortened branch, he explains this shortening as an
inevitable physiological fact, since the cost of the development of the
reproductive elements is so great as necessarily to check vegetative
growth. In the same manner the shortening of the inflorescence from
raceme to spike or umbel, and thence to the capitulum or dense
flower-head of the composite plants is brought about. This shortening,
carried still further, produces the flattened leaf-like receptacle of
Dorstenia, and further still the deeply hollowed fruity receptacle of
the fig.
The flower itself undergoes a parallel modification due to a similar
cause. It is formed by a series of modified leaves arranged round a
shortened axis. In its earlier stages the number of these modified
leaves is indefinite, as in many Ranunculaceae; and the axis itself is
not greatly shortened, as in Myosurus. The first advance is to a
definite number of parts and a permanently shortened axis, in the
arrangement termed hypogynous, in which all the whorls are quite
distinct from each other. In the next stage there is a further
shortening of the central axis, leaving the outer portion as a ring on
which the petals are inserted, producing the arrangement termed
perigynous. A still further advance is made by the contraction of the
axis, so as to leave the central part forming the ovary quite below the
flower, which is then termed epigynous.
These several modifications are said to be parallel and definite, and to
be determined by the continuous checking of vegetation by reproduction
along what is an absolute groove of progressive change. This being the
case, the importance of natural selection is greatly diminished. Instead
of selecting and accumulating spontaneous indefinite variations, its
function is to retard them after the stage of maximum utility has been
independently reached. The same simple conception is said to unlock
innumerable problems of vegetable morphology, large and small alike. It
explains the inevitable development of gymnosperm into angiosperm by the
checked vegetative growth of the ovule-bearing leaf or carpel; while
such minor adaptations as the splitting fruit of the geranium or the
cupped stigma of the pansy, can be no longer looked upon as achievements
of natural selection, but must be regarded as naturally traceable to
the vegetative checking of their respective types of leaf organ. Again,
a detailed examination of spiny plants practically excludes the
hypothesis of mammalian selection altogether, and shows spines to arise
as an expression of the diminishing vegetativeness--in fact, the ebbing
vitality of a species.[209]
_Objections to the Theory._
The theory here sketched out is enticing, and at first sight seems
calculated to throw much light on the history of plant development; but
on further consideration, it seems wanting in definiteness, while it is
beset with difficulties at every step. Take first the shortening of the
raceme into the umbel and the capitulum, said to be caused by arrest of
vegetative growth, due to the antagonism of reproduction. If this were
the whole explanation of the phenomenon, we should expect the quantity
of seed to increase as this vegetative growth diminished, since the seed
is the product of the reproductive energy of the plant, and its quantity
the best measure of that energy. But is this the case? The ranunculus
has comparatively few seeds, and the flowers are not numerous; while in
the same order the larkspur and the columbine have far more seeds as
well as more flowers, but there is no shortening of the raceme or
diminution of the foliage, although the flowers are large and complex.
So, the extremely shortened and compressed flower-heads of the
compositae produce comparatively few seeds--one only to each flower;
while the foxglove, with its long spike of showy flowers, produces an
enormous number.
Again, if the shortening of the central axis in the successive stages of
hypogynous, perigynous, and epigynous flowers were an indication of
preponderant reproduction and diminished vegetation, we should find
everywhere some clear indications of this fact. The plants with
hypogynous flowers should, as a rule, have less seed and more vigorous
and abundant foliage than those at the other extreme with epigynous
flowers. But the hypogynous poppies, pinks, and St. John's worts have
abundance of seed and rather scanty foliage; while the epigynous
dogwoods and honeysuckles have few seeds and abundant foliage. If,
instead of the number of the seeds, we take the size of the fruit as an
indication of reproductive energy, we find this at a maximum in the
gourd family, yet their rapid and luxuriant growth shows no diminution
of vegetative power. So that the statement that plant modifications
proceed "along an absolute groove of progressive change" is contradicted
by innumerable facts indicating advance and regression, improvement or
degradation, according as the ever-changing environment renders one form
more advantageous than the other. As one instance I may mention the
Anonaceae or custard-apple tribe, which are certainly an advance from
the Ranunculaceae; yet in the genus Polyalthea the fruit consists of a
number of separate carpels, each borne on a long stalk, as if reverting
to the primitive stalked carpellary leaves.
_On the Origin of Spines._
But perhaps the most extraordinary application of the theory is that
which considers spines to be an indication of the "ebbing vitality of a
species," and which excludes "mammalian selection altogether." If this
were true, spines should occur mainly in feeble, rare, and dying-out
species, instead of which we have the hawthorn, one of our most vigorous
shrubs or trees, with abundant vitality and an extensive range over the
whole Palaearctic region, showing that it is really a dominant species.
In North America the numerous thorny species of Crataegus are equally
vigorous, as are the false acacia (Robinia) and the honey-locust
(Gleditschia). Neither have the numerous species of very spiny Acacias
been noticed to be rarer or less vigorous than the unarmed kinds.
On the other point--that spines are not due to mammalian selection--we
are able to adduce what must be considered direct and conclusive
evidence. For if spines, admittedly produced by aborted branches,
petioles, or peduncles, are due solely or mainly to diminished
vegetativeness or ebbing vitality, they ought to occur in all countries
alike, or at all events in all whose similar conditions tend to check
vegetation; whereas, if they are, solely or mainly, developed as a
protection against the attacks of herbivorous mammals, they ought to be
most abundant where these are plentiful, and rare or absent where
indigenous mammalia are wanting. Oceanic islands, as compared with
continents, would thus furnish a crucial test of the two theories; and
Mr. Hemsley of Kew, who has specially studied insular floras, has given
me some valuable information on this point. He says: "There are no spiny
or prickly plants in the indigenous element of the St. Helena flora. The
relatively rich flora of the Sandwich Isles is not absolutely without a
prickly plant, but almost so. All the endemic genera are unarmed, and
the endemic species of almost every other genus. Even such genera as
Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum, of which there are
many armed species in other countries, are only represented by unarmed
species. The two endemic Rubi have the prickles reduced to the setaceous
condition, and the two palms are unarmed.
"The flora of the Galapagos includes a number of prickly plants, among
them several cacti (these have not been investigated and may be American
species), but I do not think one of the known endemic species of any
family is prickly or spiny.
"Spiny and prickly plants are also rare in New Zealand, but there are
the formidably armed species of wild Spaniard (Aciphylla), one species
of Rubus, the pungent-leaved Epacrideae and a few others."
Mr. J.G. Baker of Kew, who has specially studied the flora of Mauritius
and the adjacent islands, also writes me on this point. He says: "Taking
Mauritius alone, I do not call to mind a single species that is a
spinose endemic tree or shrub. If you take the whole group of islands
(Mauritius, Bourbon, Seychelles, and Rodriguez), there will be about a
dozen species, but then nine of these are palms. Leaving out palms, the
trees and shrubs of that part of the world are exceptionally
non-spinose."
These are certainly remarkable facts, and quite inexplicable on the
theory of spines being caused solely by checked vegetative growth, due
to weakness of constitution or to an arid soil and climate. For the
Galapagos and many parts of the Sandwich Islands are very arid, as is a
considerable part of the North Island of New Zealand. Yet in our own
moist climate and with our very limited number of trees and shrubs we
have about eighteen spiny or prickly species, more, apparently, than in
the whole endemic floras of the Mauritius, Sandwich Islands, and
Galapagos, though these are all especially rich in shrubby and arboreal
species. In New Zealand the prickly Rubus is a leafless trailing plant,
and its prickles are probably a protection against the large snails of
the country, several of which have shells from two to three and a half
inches long.[210] The "wild Spaniards" are very spiny herbaceous
Umbelliferae, and may have gained their spines to preserve them from
being trodden down or eaten by the Moas, which, for countless ages, took
the place of mammals in New Zealand. The exact use or meaning of the
spines in palms is more doubtful, though they are, no doubt, protective
against some animals; but it is certainly an extraordinary fact that in
the entire flora of the Mauritius, so largely consisting of trees and
shrubs, not a single endemic species should be thorny or spiny.
If now we consider that every continental flora produces a considerable
proportion of spiny and thorny species, and that these rise to a maximum
in South Africa, where herbivorous mammalia were (before the settlement
of the country), perhaps, more abundant and varied than in any other
part of the world; while another district, remarkable for well-armed
vegetation, is Chile, where the camel-like vicugnas, llamas, and
alpacas, and an abundance of large rodents wage perpetual war against
shrubby vegetation, we shall see the full significance of the almost
total absence of thorny and spiny plants in the chief oceanic islands;
and so far from "excluding the hypothesis of mammalian selection
altogether," we shall find in this hypothesis the only satisfactory
explanation of the facts.
From the brief consideration of Professor Geddes's theory now given, we
conclude that, although the antagonism between vegetative and
reproductive growth is a real agency, and must be taken account of in
our endeavour to explain many of the fundamental facts in the structure
and form of plants, yet it is so overpowered and directed at every step
by the natural selection of favourable variations, that the results of
its exclusive and unmodified action are nowhere to be found in nature.
It may be allowed to rank as one of those "laws of growth," of which so
many have now been indicated, and which were always recognised by Darwin
as underlying all variation; but unless we bear in mind that its action
must always be subordinated to natural selection, and that it is
continually checked, or diverted, or even reversed by the necessity of
adaptation to the environment, we shall be liable to fall into such
glaring errors as the imputing to "ebbing vitality" alone such a
widespread phenomenon as the occurrence of spines and thorns, while
ignoring altogether the influence of the organic environment in their
production.[211]
The sketch now given of the chief attempts that have been made to prove
that either the direct action of the environment or certain fundamental
laws of variation are independent causes of modification of species,
shows us that their authors have, in every case, failed to establish
their contention. Any direct action of the environment, or any
characters acquired by use or disuse, can have no effect whatever upon
the race unless they are inherited; and that they are inherited in any
case, except when they directly affect the reproductive cells, has not
been proved. On the other hand, as we shall presently show, there is
much reason for believing that such acquired characters are in their
nature non-heritable.
_Variation and Selection Overpower the Effects of Use and Disuse._
But there is another objection to this theory arising from the very
nature of the effects produced. In each generation the effects of use or
disuse, or of effort, will certainly be very small, while of this small
effect it is not maintained that the whole will be always inherited by
the next generation. How small the effect is we have no means of
determining, except in the case of disuse, which Mr. Darwin investigated
carefully. He found that in twelve fancy breeds of pigeons, which are
often kept in aviaries, or if free fly but little, the sternum had been
reduced by about one-seventh or one-eighth of its entire length, and
that of the scapula about one-ninth. In domestic ducks the weight of the
wing-bones in proportion to that of the whole skeleton had decreased
about one-tenth. In domestic rabbits the bones of the legs were found to
have increased in weight in due proportion to the increased weight of
the body, but those of the hind legs were rather less in proportion to
those of the fore legs than in the wild animal, a difference which may
be imputed to their being less used in rapid motion. The pigeons,
therefore, afford the greatest amount of reduction by
disuse--one-seventh of the length of the sternum. But the pigeon has
certainly been domesticated four or five thousand years; and if the
reduction of the wings by disuse has only been going on for the last
thousand years, the amount of reduction in each generation would be
absolutely imperceptible, and quite within the limits of the reduction
due to the absence of selection, as already explained. But, as we have
seen in Chapter III, the fortuitous variation of every part or organ
usually amounts to one-tenth, and often to one-sixth of the average
dimensions--that is, the fortuitous variation in one generation among a
limited number of the individuals of a species is as great as the
cumulative effects of disuse in a thousand generations! If we assume
that the effects of use or of effort in the individual are equal to the
effects of disuse, or even ten or a hundred times greater, they will
even then not equal, in each generation, the amount of the fortuitous
variations of the same part. If it be urged that the effects of use
would modify all the individuals of a species, while the fortuitous
variations to the amount named only apply to a portion of them, it may
be replied, that that portion is sufficiently large to afford ample
materials for selection, since it often equals the numbers that can
annually survive; while the recurrence in each successive generation of
a like amount of variation would render possible such a rapid adjustment
to new conditions that the effects of use or disuse would be as nothing
in comparison. It follows, that even admitting the modifying effects of
the environment, and that such modifications are inherited, they would
yet be entirely swamped by the greater effects of fortuitous variation,
and the far more rapid cumulative results of the selection of such
variations.
_Supposed Action of the Environment in Initiating Variations._
It is, however, urged that the reaction of the environment initiates
variations, which without it would never arise; such, for instance, as
the origin of horns through the pressures and irritations caused by
butting, or otherwise using the head as a weapon or for defence.
Admitting, for the sake of argument, that this is so, all the evidence
we possess shows that, from the very first appearance of the rudiment of
such an organ, it would vary to a greater extent than the amount of
growth directly produced by use; and these variations would be subject
to selection, and would thus modify the organ in ways which use alone
would never bring about. We have seen that this has been the case with
the branching antlers of the stag, which have been modified by
selection, so as to become useful in other ways than as a mere weapon;
and the same has almost certainly been the case with the variously
curved and twisted horns of antelopes. In like manner, every conceivable
rudiment would, from its first appearance, be subject to the law of
variation and selection, to which, thenceforth, the direct effect of the
environment would be altogether subordinate.
A very similar mode of reasoning will apply to the other branch of the
subject--the initiation of structures and organs by the action of the
fundamental laws of growth. Admitting that such laws have determined
some of the main divisions of the animal and vegetable kingdom, have
originated certain important organs, and have been the fundamental cause
of certain lines of development, yet at every step of the process these
laws must have acted in entire subordination to the law of natural
selection. No modification thus initiated could have advanced a single
step, unless it were, on the whole, a useful modification; while its
entire future course would be necessarily subject to the laws of
variation and selection, by which it would be sometimes checked,
sometimes hastened on, sometimes diverted to one purpose, sometimes to
another, according as the needs of the organism, under the special
conditions of its existence, required such modification. We need not
deny that such laws and influences may have acted in the manner
suggested, but what we do deny is that they could possibly escape from
the ever-present and all-powerful modifying effects of variation and
natural selection.[212]
_Weismann's Theory of Heredity._
Professor August Weismann has put forth a new theory of heredity founded
upon the "continuity of the germ-plasm," one of the logical consequences
of which is, that acquired characters of whatever kind are not
transmitted from parent to offspring. As this is a matter of vital
importance to the theory of natural selection, and as, if well founded,
it strikes away the foundations of most of the theories discussed in the
present chapter, a brief outline of Weismann's views must be attempted,
although it is very difficult to make them intelligible to persons
unfamiliar with the main facts of modern embryology.[213]
The problem is thus stated by Weismann: "How is it that in the case of
all higher animals and plants a single cell is able to separate itself
from amongst the millions of most various kinds of which an organism is
composed, and by division and complicated differentiation to reconstruct
a new individual with marvellous likeness, unchanged in many cases even
throughout whole geological periods?" Darwin attempted to solve the
problem by his theory of "Pangenesis," which supposed that every
individual cell in the body gave off gemmules or germs capable of
reproducing themselves, and that portions of these germs of each of the
almost infinite number of cells permeate the whole body and become
collected in the generative cells, and are thus able to reproduce the
whole organism. This theory is felt to be so ponderously complex and
difficult that it has met with no general acceptance among
physiologists.
The fact that the germ-cells _do_ reproduce with wonderful accuracy not
only the general characters of the species, but many of the individual
characteristics of the parents or more remote ancestors, and that this
process is continued from generation to generation, can be accounted
for, Weismann thinks, only on two suppositions which are physiologically
possible. Either the substance of the parent germ-cell, after passing
through a cycle of changes required for the construction of a new
individual, possesses the capability of producing anew germ-cells
identical with those from which that individual was developed, or _the
new germ-cells arise, as far as their essential and characteristic
substance is concerned, not at all out of the body of the individual,
but direct from the parent germ-cell_. This latter view Weismann holds
to be the correct one, and, on this theory, heredity depends on the fact
that a substance of special molecular composition passes over from one
generation to another. This is the "germ-plasm," the power of which to
develop itself into a perfect organism depends on the extraordinary
complication of its minutest structure. At every new birth a portion of
the specific germ-plasm, which the parent egg-cell contains, is not used
up in producing the offspring, but is reserved unchanged to produce the
germ-cells of the following generation. Thus the germ-cells--so far as
regards their essential part the germ-plasm--are not a product of the
body itself, but are related to one another in the same way as are a
series of generations of unicellular organisms derived from one another
by a continuous course of simple division. Thus the question of heredity
is reduced to one of growth. A minute portion of the very same
germ-plasm from which, first the germ-cell, and then the whole organism
of the parent, were developed, becomes the starting-point of the growth
of the child.
_The Cause of Variation._
But if this were all, the offspring would reproduce the parent exactly,
in every detail of form and structure; and here we see the importance of
sex, for each new germ grows out of the united germ-plasms of two
parents, whence arises a mingling of their characters in the offspring.
This occurs in each generation; hence every individual is a complex
result reproducing in ever-varying degrees the diverse characteristics
of his two parents, four grandparents, eight great-grandparents, and
other more remote ancestors; and that ever-present individual variation
arises which furnishes the material for natural selection to act upon.
Diversity of sex becomes, therefore, of primary importance as _the cause
of variation_. Where asexual generation prevails, the characteristics of
the individual alone are reproduced, and there are thus no means of
effecting the change of form or structure required by changed conditions
of existence. Under such changed conditions a complex organism, if only
asexually propagated, would become extinct. But when a complex organism
is sexually propagated, there is an ever-present cause of change which,
though slight in any one generation, is cumulative, and under the
influence of selection is sufficient to keep up the harmony between the
organism and its slowly changing environment.[214]
_The Non-Heredity of Acquired Characters._
Certain observations on the embryology of the lower animals are held to
afford direct proof of this theory of heredity, but they are too
technical to be made clear to ordinary readers. A logical result of the
theory is the impossibility of the transmission of acquired characters,
since the molecular structure of the germ-plasm is already determined
within the embryo; and Weismann holds that there are no facts which
really prove that acquired characters can be inherited, although their
inheritance has, by most writers, been considered so probable as hardly
to stand in need of direct proof.
We have already shown, in the earlier part of this chapter, that many
instances of change, imputed to the inheritance of acquired variations,
are really cases of selection; while the very fact that _use_ implies
_usefulness_ renders it almost impossible to eliminate the action of
selection in a state of nature. As regards mutilations, it is generally
admitted that they are not hereditary, and there is ample evidence on
this point. When it was the fashion to dock horses' tails, it was not
found that horses were born with short tails; nor are Chinese women born
with distorted feet; nor are any of the numerous forms of racial
mutilation in man, which have in some cases been carried on for hundreds
of generations, inherited. Nevertheless, a few cases of apparent
inheritance of mutilations have been recorded,[215] and these, if
trustworthy, are difficulties in the way of the theory. The undoubted
inheritance of disease is hardly a difficulty, because the
predisposition to disease is a congenital, not an acquired character,
and as such would be the subject of inheritance. The often-quoted case
of a disease induced by mutilation being inherited (Brown-Sequard's
epileptic guinea-pigs) has been discussed by Professor Weismann, and
shown to be not conclusive. The mutilation itself--a section of certain
nerves--was never inherited, but the resulting epilepsy, or a general
state of weakness, deformity, or sores, was sometimes inherited. It is,
however, possible that the mere injury introduced and encouraged the
growth of certain microbes, which, spreading through the organism,
sometimes reached the germ-cells, and thus transmitted a diseased
condition to the offspring. Such a transference of microbes is believed
to occur in syphilis and tuberculosis, and has been ascertained to occur
in the case of the muscardine silkworm disease.[216]
_The Theory of Instinct._
The theory now briefly outlined cannot be said to be proved, but it
commends itself to many physiologists as being inherently probable, and
as furnishing a good working hypothesis till displaced by a better. We
cannot, therefore, accept any arguments against the agency of natural
selection which are based upon the opposite and equally unproved theory
that acquired characters are inherited; and as this applies to the whole
school of what may be termed Neo-Lamarckians, their speculations cease
to have any weight.
The same remark applies to the popular theory of instincts as being
inherited habits; though Darwin gave very little weight to this, but
derived almost all instincts from spontaneous useful variations which,
like other spontaneous variations, are of course inherited. At first
sight it appears as if the acquired habits of our trained
dogs--pointers, retrievers, etc.--are certainly inherited; but this need
not be the case, because there must be some structural or psychical
peculiarities, such as modifications in the attachments of muscles,
increased delicacy of smell or sight, or peculiar likes and dislikes,
which are inherited; and from these, peculiar habits follow as a natural
consequence, or are easily acquired. Now, as selection has been
constantly at work in improving all our domestic animals, we have
unconsciously modified the structure, while preserving only those
animals which best served our purpose in their peculiar faculties,
instincts, or habits.
Much of the mystery of instinct arises from the persistent refusal to
recognise the agency of imitation, memory, observation, and reason as
often forming part of it. Yet there is ample evidence that such agency
must be taken into account. Both Wilson and Leroy state that young birds
build inferior nests to old ones, and the latter author observes that
the best nests are made by birds whose young remain longest in the nest.
So, migration is now well ascertained to be effected by means of vision,
long flights being made on bright moonlight nights when the birds fly
very high, while on cloudy nights they fly low, and then often lose
their way. Thousands annually fly out to sea and perish, showing that
the instinct to migrate is imperfect, and is not a good substitute for
reason and observation.
Again, much of the perfection of instinct is due to the extreme severity
of the selection during its development, any failure involving
destruction. The chick which cannot break the eggshell, the caterpillar
that fails to suspend itself properly or to spin a safe cocoon, the bees
that lose their way or that fail to store honey, inevitably perish. So
the birds that fail to feed and protect their young, or the butterflies
that lay their eggs on the wrong food-plant, leave no offspring, and the
race with imperfect instincts perishes. Now, during the long and very
slow course of development of each organism, this rigid selection at
every step of progress has led to the preservation of every detail of
structure, faculty, or habit that has been necessary for the
preservation of the race, and has thus gradually built up the various
instincts which seem so marvellous to us, but which can yet be shown to
be in many cases still imperfect. Here, as everywhere else in nature, we
find comparative, not absolute perfection, with every gradation from
what is clearly due to imitation or reason up to what seems to us
perfect instinct--that in which a complex action is performed without
any previous experience or instruction.[217]
_Concluding Remarks._
Having now passed in review the more important of the recent objections
to, or criticisms of, the theory of natural selection, we have arrived
at the conclusion that in no one case have the writers in question been
able materially to diminish its importance, or to show that any of the
laws or forces to which they appeal can act otherwise than in strict
subordination to it. The direct action of the environment as set forth
by Mr. Herbert Spencer, Dr. Cope, and Dr. Karl Semper, even if we admit
that its effects on the individual are transmitted by inheritance, are
so small in comparison with the amount of spontaneous variation of every
part of the organism that they must be quite overshadowed by the latter.
And if such direct action may, in some cases, have initiated certain
organs or outgrowths, these must from their very first beginnings have
been subject to variation and natural selection, and their further
development have been almost wholly due to these ever-present and
powerful causes. The same remark applies to the views of Professor
Geddes on the laws of growth which have determined certain essential
features in the morphology of plants and animals. The attempt to
substitute these laws for those of variation and natural selection has
failed in cases where we can apply a definite test, as in that of the
origin of spines on trees and shrubs; while the extreme diversity of
vegetable structure and form among the plants of the same country and of
the same natural order, of itself affords a proof of the preponderating
influence of variation and natural selection in keeping the many diverse
forms in harmony with the highly complex and ever-changing environment.
Lastly, we have seen that Professor Weismann's theory of the continuity
of the germ-plasm and the consequent non-heredity of acquired
characters, while in perfect harmony with all the well-ascertained facts
of heredity and development, adds greatly to the importance of natural
selection as the one invariable and ever-present factor in all organic
change, and that which can alone have produced the temporary fixity
combined with the secular modification of species. While admitting, as
Darwin always admitted, the co-operation of the fundamental laws of
growth and variation, of correlation and heredity, in determining the
direction of lines of variation or in the initiation of peculiar organs,
we find that variation and natural selection are ever-present agencies,
which take possession, as it were, of every minute change originated by
these fundamental causes, check or favour their further development, or
modify them in countless varied ways according to the varying needs of
the organism. Whatever other causes have been at work, Natural Selection
is supreme, to an extent which even Darwin himself hesitated to claim
for it. The more we study it the more we are convinced of its
overpowering importance, and the more confidently we claim, in Darwin's
own words, that it "has been the most important, but not the exclusive,
means of modification."
FOOTNOTES:
[Footnote 198: See the Duke of Argyll's letter in _Nature_, vol. xxxiv.
p. 336.]
[Footnote 199: _Journal of the Anthropological Institute,_ vol. xv. pp.
246-260.]
[Footnote 200: The idea of the non-heredity of acquired variations was
suggested by the summary of Professor Weismann's views, in _Nature_,
referred to later on. But since this chapter was written I have, through
the kindness of Mr. E.B. Poulton, seen some of the proofs of the
forthcoming translation of Weismann's Essays on Heredity, in which he
sets forth an explanation very similar to that here given. On the
difficult question of the almost entire disappearance of organs, as in
the limbs of snakes and of some lizards, he adduces "a certain form of
correlation, which Roux calls 'the struggle of the parts in the
organism,'" as playing an important part. Atrophy following disuse is
nearly always attended by the corresponding increase of other organs:
blind animals possess more developed organs of touch, hearing, and
smell; the loss of power in the wings is accompanied by increased
strength of the legs, etc. Now as these latter characters, being useful,
will be selected, it is easy to understand that a congenital increase of
these will be accompanied by a corresponding congenital diminution of
the unused organ; and in cases where the means of nutrition are
deficient, every diminution of these useless parts will be a gain to the
whole organism, and thus their complete disappearance will, in some
cases, be brought about directly by natural selection. This corresponds
with what we know of these rudimentary organs.
It must, however, be pointed out that the non-heredity of acquired
characters was maintained by Mr. Francis Galton more than twelve years
ago, on theoretical considerations almost identical with those urged by
Professor Weismann; while the insufficiency of the evidence for their
hereditary transmission was shown, by similar arguments to those used
above and in the work of Professor Weismann already referred to (see "A
Theory of Heredity," in _Journ. Anthrop. Instit._, vol. v. pp.
343-345).]
[Footnote 201: This explanation is derived from Weismann's Theory of the
Continuity of the Germ-Plasm as summarised in _Nature_.]
[Footnote 202: See a collection of his essays under the title, _The
Origin of the Fittest: Essays on Evolution_, D. Appleton and Co. New
York. 1887.]
[Footnote 203: _Origin of the Fittest_, p. 174.]
[Footnote 204: _Ibid._ p. 29. It may be here noted that Darwin found
these theories unintelligible. In a letter to Professor E.T. Morse in
1877, he writes: "There is one point which I regret you did not make
clear in your Address, namely, what is the meaning and importance of
Professors Cope and Hyatt's views on acceleration and retardation? I
have endeavoured, and given up in despair, the attempt to grasp their
meaning" (_Life and Letters_, vol. iii. p. 233).]
[Footnote 205: _Origin of the Fittest_, p. 374.]
[Footnote 206: _Origin of the Fittest_, p. 40.]
[Footnote 207: _The Natural Conditions of Existence as they Affect
Animal Life._ London, 1883.]
[Footnote 208: In Dr. Weismann's essay on "Heredity," already referred
to, he considers it not improbable that changes in organisms produced by
climatic influences may be inherited, because, as these changes do not
affect the external parts of an organism only, but often, as in the case
of warmth or moisture permeate the whole structure, they may possibly
modify the germ-plasm itself, and thus induce variations in the next
generation. In this way, he thinks, may possibly be explained the
climatic varieties of certain butterflies, and some other changes which
seem to be effected by change of climate in a few generations.]
[Footnote 209: This brief indication of Professor Geddes's views is
taken from the article "Variation and Selection" in the _Encyclopedia
Britannica_, and a paper "On the Nature and Causes of Variation in
Plants" in _Trans. and Proc. of the Edinburgh Botanical Society_, 1886;
and is, for the most part, expressed in his own words.]
[Footnote 210: Placostylis bovinus, 3½ inches long; Paryphanta Busbyi, 3
in. diam.; P. Hochstetteri, 2¾ in. diam.]
[Footnote 211: The general arguments and objections here set forth will
apply with equal force to Professor G. Henslow's theory of the origin of
the various forms and structures of flowers as due to "the responsive
actions of the protoplasm in consequence of the irritations set up by
the weights, pressures, thrusts, tensions, etc., of the insect visitors"
(_The Origin of Floral Structures through Insect and other Agencies_, p.
340). On the assumption that acquired characters are inherited, such
irritations may have had something to do with the initiation of
variations and with the production of certain details of structure, but
they are clearly incompetent to have brought about the more important
structural and functional modifications of flowers. Such are, the
various adjustments of length and position of the stamens to bring the
pollen to the insect and from the insect to the stigma; the various
motions of stamens and styles at the right time and the right direction;
the physiological adjustments bringing about fertility or sterility in
heterostyled plants; the traps, springs, and complex movements of
various parts of orchids; and innumerable other remarkable phenomena.
For the explanation of these we have no resource but variation and
selection, to the effects of which, acting alternately with regression
or degradation as above explained (p. 328) must be imputed the
development of the countless floral structures we now behold. Even the
primitive flowers, whose initiation may, perhaps, have been caused, or
rendered possible, by the irritation set up by insects' visits, must,
from their very origin, have been modified, in accordance with the
supreme law of utility, by means of variation and survival of the
fittest.]
[Footnote 212: In an essay on "The Duration of Life," forming part of
the translation of Dr. Weismann's papers already referred to, the author
still further extends the sphere of natural selection by showing that
the average duration of life in each species has been determined by it.
A certain length of life is essential in order that the species may
produce offspring sufficient to ensure its continuance under the most
unfavourable conditions; and it is shown that the remarkable
inequalities of longevity in different species and groups may be thus
accounted for. Yet more, the occurrence of death in the higher
organisms, in place of the continued survival of the unicellular
organisms however much they may increase by subdivision, may be traced
to the same great law of utility for the race and survival of the
fittest. The whole essay is of exceeding interest, and will repay a
careful perusal. A similar idea occurred to the present writer about
twenty years back, and was briefly noted down at the time, but
subsequently forgotten.]
[Footnote 213: The outline here given is derived from two articles in
_Nature_, vol. xxxiii. p. 154, and vol. xxxiv. p. 629, in which
Weismann's papers are summarised and partly translated.]
[Footnote 214: There are many indications that this explanation of the
cause of variation is the true one. Mr. E.B. Poulton suggests one, in
the fact that parthenogenetic reproduction only occurs in isolated
species, not in groups of related species; as this shows that
parthenogenesis cannot lead to the evolution of new forms. Again, in
parthenogenetic females the complete apparatus for fertilisation remains
unreduced; but if these varied as do sexually produced animals, the
organs referred to, being unused, would become rudimentary.
Even more important is the significance of the "polar bodies," as
explained by Weismann in one of his _Essays_; since, if his
interpretation of them be correct, variability is a necessary
consequence of sexual generation.]
[Footnote 215: Darwin's _Animals and Plants_, vol. ii. pp. 23, 24.]
[Footnote 216: In his essay on "Heredity," Dr. Weismann discusses many
other cases of supposed inheritance of acquired characters, and shows
that they can all be explained in other ways. Shortsightedness among
civilised nations, for example, is due partly to the absence of
selection and consequent regression towards a mean, and partly to its
individual production by constant reading.]
[Footnote 217: Weismann explains instinct on similar lines, and gives
many interesting illustrations (see _Essays on Heredity_). He holds
"that all instinct is entirely due to the operation of natural
selection, and has its foundation, not upon inherited experiences, but
upon variations of the germ." Many interesting and difficult cases of
instinct are discussed by Darwin in Chapter VIII of the _Origin of
Species_, which should be read in connection with the above remarks.
Since this chapter was written my attention has been directed to Mr.
Francis Galton's _Theory of Heredity_ (already referred to at p. 417)
which was published thirteen years ago as an alternative for Darwin's
theory of pangenesis.
Mr. Galton's theory, although it attracted little attention, appears to
me to be substantially the same as that of Professor Weismann. Galton's
"stirp" is Weismann's "germ-plasm." Galton supposes the sexual elements
in the offspring to be directly formed from the residue of the _stirp_
not used up in the development of the body of the parent--Weismann's
"continuity of the germ-plasm." Galton also draws many of the same
conclusions from his theory. He maintains that characters acquired by
the individual as the result of external influences cannot be inherited,
unless such influences act directly on the reproductive
elements--instancing the possible heredity of alcoholism, because the
alcohol permeates the tissues and may reach the sexual elements. He
discusses the supposed heredity of effects produced by use or disuse,
and explains them much in the same manner as does Weismann. Galton is an
anthropologist, and applies the theory, mainly, to explain the
peculiarities of hereditary transmission in man, many of which
peculiarities he discusses and elucidates. Weismann is a biologist, and
is mostly concerned with the application of the theory to explain
variation and instinct, and to the further development of the theory of
evolution. He has worked it out more thoroughly, and has adduced
embryological evidence in its support; but the views of both writers are
substantially the same, and their theories were arrived at quite
independently. The names of Galton and Weismann should therefore be
associated as discoverers of what may be considered (if finally
established) the most important contribution to the evolution theory
since the appearance of the _Origin of Species_.]