THE MECHANICAL THEORY OF EVOLUTION: THE DARWIN-LAMARCK EXPLANATION
“Quelle est donc cette nature sujette à être effacée? La coutume
est une seconde nature qui detruit la première. Pourquoi la coutume
n’est elle pas naturelle? J’ai bien peur que cette nature ne soit
elle-même qu’une première coutume, comme la coutume est une seconde
nature.”—Pascal.
We now approach the _arcana_ of Evolution. The processes we have to
deal with in this chapter are not, and probably never will be, the
subjects of direct observation. All we can hope to do is to generalize
from the results which have risen to the surface of life about the
unseen forces from which they spring. The problem is to find (if
possible) a generalization which will cover all the facts relating to
that modification of natural forms, habits, and instincts which, when
it reaches a certain point, means the establishment of a new species.
We know that the thing happens, but we shall not understand _how_ it
happens until either the mechanism of the process is laid bare, or
until it is clear that we are in presence of an agency not entirely
definable in terms of mechanical action.
The fixity of species is maintained by a number of conditions, chief
among which must be reckoned the law of reproduction by conjugation,
with the consequent intermixture of numerous different lines of
descent. From one point of view conjugation, as Weismann so often
insists, greatly favours the adaptability of the organism to new and
varied conditions of life, inasmuch as it results in the mingling
together in each individual of a great number of varied determinants.
But when the conditions are constant, conjugation has also the
obvious effect of constantly reabsorbing, as it were, any heritable
abnormalities which may occur in individuals or the species, and
bringing them back to type. An individual possessing some abnormality
of structure will be most unlikely to find a mate possessing the same
abnormality—the mate will be either an ordinary individual or will
possess, if any, some quite different variation. Their descendants
will, therefore, usually show more resemblance to the normal type
than to the one abnormal parent, and in their descendants again, for
the same reason, the abnormal feature will be still further reduced,
until finally it disappears. It is only by the careful selection of
mates extending over many generations that pigeon-fanciers, to take
one prominent instance, are able to establish a new type. Left to mate
uncontrolled among themselves we should never have had the great
variety of breeds which have been produced by the art of the fancier
from the original rock-pigeon. The small variations which form the
starting points of his operations would, under natural conditions, have
soon been resolved into the normal type. What is it in nature, then,
that sometimes appears to play the part of the intelligent breeder and
to urge the plastic forms of life into new moulds?
The goal of the breeder is some new form which it pleases him
to produce, either for its use, or its beauty, or for its mere
singularity. The goal of nature, at least the apparent and immediate
goal, is the adaptation of each species to the circumstances of its
life. And the first thing that strikes the investigator is the way,
often indeed not perfect, but usually most impressive in its apparent
thoughtfulness and care, in which the organs of plants and animals are
fashioned to secure the most favourable results. But all this is the
result of development. The whale is a creature excellently adapted for
its present mode of life, but we know that it was once a furry land
animal with four legs; the legs are all there still, in modified or
rudimentary form, and the fur appears at a certain stage of embryonic
development. When we ask, How did this extraordinary transformation
come about? what we really mean is, How did the determinants composing
the chromatin in the reproductive cells of the original land animal
so come to alter as to produce the characteristics of the whale? For
new species can only be evolved by means of structural modifications
_capable of being transmitted by inheritance_; and nothing can be
inherited except through the action of the determinants. A modification
which does not affect the reproductive cells has no significance in the
evolution of species.
To this question Darwinism has given us our choice of two answers,
which may be termed respectively the Darwin-Lamarck and the
Darwin-Weismann theories. Lamarck explained the origin of species by
the accumulated effect of the inheritance, through many generations, of
modifications acquired by the exercise, or the disuse, of the modified
organs. Observing that living protoplasm responds to demands upon it
(thus, for instance, a muscle when systematically exercised attracts
more nourishment from the blood and grows stronger, and callosities
form to protect the skin of the hands of a manual worker), he assumed
that modifications so acquired might be transmitted by inheritance.
Each new generation, then, would start with a slightly better equipment
in this particular respect than the former one had when it started;
and so, by slow degrees, a new organ, or one markedly differing from
the original form, might be built up. The world, since protoplasmic
life first appeared upon it, has gone through many changes, and has
always presented a vast variety of climatic and other conditions,
calling for the most varied types of organic structure. As animal
life gradually spread over the earth and sea, the effort to cope with
the different conditions it met with would gradually, by the combined
action of exercise, of disuse, and of heredity, produce multitudes
of different types; and these are what we know as families, orders,
genera, and species. When a species is fairly well adapted to its
surroundings and way of life it may go on indefinitely without change.
But should any members of it be obliged to migrate, from scarcity
of food or any other reason, to some new locality where somewhat
different conditions prevail, structural alterations would soon begin
to appear to suit those new conditions. Thus the giraffe, if we could
trace its ancestry back, would probably be found to have originated in
some animal not differing from the vast majority of quadrupeds in the
relative proportions of its fore and hind quarters. But some members
of this original species—or the whole species, owing to some change
in their surroundings—found themselves obliged to rely largely for
food on leaves growing at a considerable height. They stretched up to
reach them, and a prolongation of the bones of the neck (the giraffe
has only the usual seven cervical vertebræ) and of the fore-legs would
ensue, especially in the young; this prolongation would be handed on
by inheritance, and so by degrees the new type of animal would be
evolved. The horn of the rhinoceros, the antlers of the stag, the
canine teeth of beasts of prey, the flat grinders of ruminants, the
flippers of the whale, the proboscis of the honey-feeding butterfly,
the jaws of the ant or the beetle, and a host of other adaptations
which seem obviously to owe their origin to the exercise of their
functions, occur to the mind in confirmation of this theory.
Besides Adaptation, we have what appears the strikingly confirmatory
case of what is called Co-adaptation, where the variation of one
organ or structure in an animal puts a strain upon other parts, which
accordingly respond by auxiliary adaptations. Such co-adaptations are
numerous in every animal structure, and, as we cannot suppose them to
have all originated simultaneously and by chance, the conclusion drawn
by Lamarckians is that one was produced by use, and, in the course of
its development, produced the others in the same way. A typical case is
that of the Irish elk. The enormous antlers of this beast, sometimes
weighing a hundredweight, must have needed (besides other structural
changes) a cervical ligament of immense size and power to support
them, and from the peculiar structure of the cervical vertebræ it is
demonstrable that such a ligament must have existed. What more natural
than to suppose that the antlers were developed by fighting wild beasts
of prey, combats between male elks, etc., and that then in their
gradual growth, as the species was evolved, the ligament and the bony
structure associated with it responded to the increasing strain. That
is exactly what would happen in an individual. We have only to assume
the heritability of modifications acquired by use to understand how
these co-adaptations became constant characters in a species.
Not less apparent cogency for the argument for modifications by use
have those cases where the modification has been apparently due to
disuse. It is well known that living creatures found in the total
darkness of great limestone caverns, like those at Kentucky, are
blind, through imperfections of one kind or another in the organs of
sight. But the rudimentary structures which remain tell us that these
creatures had ancestors which were once fully equipped in this respect,
and which had wandered into the caverns from the sunlit outer world.
Thus the case of a crab has been noted, in which the stalks on which
a crab’s eyes are set were preserved, while the eyes had disappeared:
it is, as Darwin observes, as if the stand of a telescope had been
retained while the telescope itself had gone. Sometimes the eyes
of cave-fishes are covered with a horny layer, sometimes the whole
structure is atrophied and withered. But never is an animal found
under these conditions which has retained its power of sight. The
conclusion seems obvious. In individuals, a muscle or other organ is
known to strengthen and develop by use and to atrophy by disuse. As use
and disuse appear to be invariably accompanied by precisely the same
effects in the species as in the individual, and as there seems no way
of accounting for this by any known physiological law without assuming
that modifications acquired by the individual are transmitted to its
progeny, the case for the inheritability of such modifications appears,
at first sight, irresistible.[54]
So matters stood when Darwin’s Origin of Species carried the argument
for evolution a long step further. Accepting fully the views of
Lamarck, Darwin attempted, by his doctrine of Natural Selection, first
to reinforce those views, secondly to explain much that they could
not be made to cover. It is plain that _if we assume the existence of
a severe competition for livelihood among the members of a species_,
any favourable variations of structure or instinct which may occur
among certain members of the species will give their type an advantage
over the normal type in the struggle for existence. They will, on the
average, live longer and produce more offspring. Ultimately, as the
struggle for life is always most severe among nearly related organisms,
which seek a living from the same sources, the less perfectly equipped
type will be extinguished, and so on, until a species exhibiting the
most complete form of adaptation has been evolved. The variations
on which Natural Selection has to work are produced, according to
Darwin, not only by the exercise of particular organs as in Lamarck’s
theory, but also and more potently by “innate variations” originating
from unascertained causes in the reproductive cells. Variations, it
is indisputable, are always occurring; probably no two members of
any species exactly resemble each other. Among low and primitive
organisms, such as the _Foraminifera_, Dr. W. B. Carpenter (I quote
from A. R. Wallace’s Darwinism) found, on careful examination, the
range of variation so great that characteristics typical not merely
of species but of genera and even of orders were liable to vary,[55]
while at the other end of evolution, in man, to give only one instance,
Mr. J. Wood is stated by Darwin to have observed no less than five
hundred and fifty-eight variations in the muscular structure of
thirty-six subjects examined.[56] The cause of these variations is
often quite obscure, but it is certain that some kinds of them are
capable of arising as the natural response of the organism to changed
conditions of food or habitat Conditions such as these, affecting
the whole constitution of the organism, have been proved capable of
affecting the reproductive cells, and thus of giving rise to hereditary
characteristics. Natural Selection, then, by preserving and encouraging
the better fitted as opposed to the less fit, acts as a spur to the
Lamarckian principles of development by exercise of function, while
it also lays hold of and intensifies all kinds of other favourable
variations occurring either casually or in consequence of change of
habitat, and weeds out the types in which such variations happen to
be unfavourable. According to Darwin, therefore, given (1) constant
variations of structure arising from use, disuse, or from other known
or unknown causes, (2) the capacity to transmit by inheritance these
variations _whether innate or acquired_, (3) a constant struggle for
existence among organisms both against each others’ competition and
against the general conditions of life[57]—given these simple data,
the secret springs of evolution are laid bare, and the vast complexity
of natural forms upon the globe is adequately accounted for without
calling in the agency of special creations. But variations are
the starting-point in the process: Natural Selection can originate
nothing—it can only act on what is presented to it by some quite
different force. The relative parts played by the various agencies at
work are, with characteristic moderation of statement, thus described
by Darwin:—
“On the whole I think we may conclude that habit, use, and disuse,
have, in some cases, played a considerable part in the modification
of the constitution, and in the structure of various organs; but
that the effects of use and disuse have often been largely combined
with, and sometimes overmastered by the natural selection of innate
variations.”[58]
To explain evolution, then, we must first explain _the occurrence
of appropriate variations_, strong enough and widespread enough
to maintain themselves against the constant reducing influence of
promiscuous intercrossing, and they must be variations _capable of
being transmitted by inheritance_. This, we now see, is the true field
of the inquiry.
The new factors introduced by Darwin into the process of
evolution—Natural Selection and Innate Variations—were destined
in our day to have the whole weight of the argument for evolution
suddenly thrown upon them. The inheritability of variations acquired
by the individual through use and disuse when subjected to fresh
investigation by the younger school of biologists has turned out to be
open to the gravest doubts, both theoretically, on account of the great
difficulty of reconciling it with what has now been ascertained of the
nature of the reproductive mechanism in plants and animals, and also on
the score of a closer consideration of the facts commonly adduced as
evidence for the law. To take these points separately: The reproductive
cells in every living creature are now believed to be formed directly
from the reproductive cells of its parents. They are not a product
of the organism in which they find themselves. They are nourished
by its blood, and are therefore liable to be affected by anything
which produces a broad general effect on the constitution of the
being in whom they are lodged, but it is difficult to see how special
modifications of individual parts of that being could affect them so
as to influence the determinants in the direction of reproducing that
modification. How, for example, could the habit of grubbing for roots
in an animal of the pig tribe so affect its reproductive cells as to
ensure the birth of an offspring with callosities on their snouts?
The physiological mechanism by which such a result could be produced
seems hardly conceivable—at any rate no one has yet offered a plausible
conception of it. Of course if the _fact_ were indisputably proved
one would only have to accept it, and endeavour, if possible, to
discover the why and how. But the fact, which once looked so solidly
established, is taking on a more and more insubstantial appearance in
the light of closer investigation.
The argument against Lamarckism rests on the basis (1) artificial
experiment, (2) of observation of nature under normal conditions.
As to the evidence from experiment, opinions fluctuated for some
time—Darwin was disposed at one time to deny, at another to admit the
alleged proofs it offered. In the present day opinion is overwhelmingly
against the validity of these proofs. The cases where artificially
produced mutilations are said to have been inherited have, when
investigated, turned out to be by no means as clear and trustworthy
as was supposed, nor can one place much reliance on a few cases of
striking coincidence such as are certain to occur from time to time.[59]
The adverse instances are very clear indeed. Chinese girls are never
born with abnormally small feet. Jews are not born circumcised. Among
tribes where tattooing is practised, no traces of this embellishment
are ever found to be inherited. If it is a physiological law that the
disuse of an organ not only atrophies it in the individual but (by
inheritance of the atrophy) eliminates it from the species, there is
no apparent reason why this law should not operate in cases where the
organ is artificially removed. Yet it rarely or never seems to do so.
Experiments upon animals, such as breeding for many generations from
mice whose tails have been cut off, have never resulted in producing a
clear case of inherited mutilation. A strong presumption is therefore
raised that the effects apparently due to use and disuse under natural
conditions (as in the eyeless fishes of the Kentucky caves) must be set
down to some other cause. The queens in colonies of ants and bees have
never exercised the functions of workers for thousands of centuries,
yet they transmit these functions unimpaired.
There is, indeed, a case often referred to in this connexion which
must be here mentioned. Dr. Brown-Séquard found that by injuring or
compressing the sciatic nerve in guinea-pigs epilepsy was produced, and
that the descendants of animals so injured had a marked tendency to
epileptic fits. This is undoubtedly a very significant and important
fact in biology, but it gives no support to the Lamarckian theory.
What is inherited by the guinea-pigs is not the injury to the nerve
but the pathological condition resulting therefrom. It remains to
be discovered how, precisely, this takes place, and the experiment
may end in illuminating a very obscure region in physiology, but on
Lamarckism it has no bearing at all. A better case is that of atrophy
of a toe, which is said to have been inherited in consequence of its
original production by severance of the sciatic nerve, but, again,
what is inherited is not an actual injury but an effect of it. It is
clear, however, that bodily conditions of a large and comprehensive
kind produced naturally or artificially in an individual may have an
effect on the reproductive cells, especially when the nervous system is
affected.
Coming to the observation of what happens under natural conditions,
we are struck at the outset by the fact that the inheritance of
acquired characteristics, if it works at all, must work under some
system of salutary control and not as a blind physiological law. For
if each generation starts with some measure at least of what the
former generation had acquired, and adds to it by its own activity,
then all acquired characteristics would ere long attain a monstrous
development, and the species would perish under them. But nothing of
the kind is observed to happen. The continual use of the muscles in the
labouring classes has not made men stronger than they were thousands
of generations ago. The habit of handling the spade and hoe has never
produced a peasant child born with callosities on its hands. The horn
of the rhinoceros, which on Lamarckian principles we must regard as
developed by the gradual increase of a callosity formed by grubbing for
roots, does not grow beyond a certain size, however the species may go
on grubbing. The Lamarckian law, then, if it has any real effect at
all, can only express half the truth about the action of heredity on
acquired characteristics. As the column of water in a fountain hovers
about a certain height, so the action of heredity in the accumulation
of the effects produced by the use of organs seems to have a limit
beyond which it cannot pass. May it not be that heredity is really as
false an expression for the phenomenon as the popular superstition
about ‘water seeking its own level’ is for the upspringing of a
fountain?
The cases of co-adaptation, where one organ appears to be developed
by use and others by the use of that, as in the case of the Irish
elk referred to above, are met by instances just as striking where
the elements of modification by use cannot come into play. Weismann
mentions the case of the ingenious brush arrangement on the anterior
legs of the bee, which the insect uses for cleansing its antennæ. Two
adaptations are here developed—a little semicircular notch in the leg,
set with small bristles, and a movable projection or flap used for
pressing the antenna into the notch as it is drawn through. The bee, no
doubt, would naturally try to clean its antennæ with its fore-legs, but
how could this process develop the special arrangements referred to in
the hard or scaly covering of its limbs? It is not until the shell of
the insect has grown quite hard and incapable of further vital changes
that the arrangement comes into use. Again, the stridulating noise
produced by the legs of the grasshopper is due to serrations occurring
on different joints of the limb. Serrations on one joint would in
no way tend to develop them on the other, but rather the contrary,
yet there they are, in harmonious co-operation. If Nature can obtain
these effects, as she does in numberless instances, without the aid of
Lamarck’s principle, we cannot help asking whether that principle is
ever operative at all.
The three instances which we shall next consider seem to offer very
serious obstacles to the Lamarckian theory.
A modification of structure caused by the special use of a certain
organ takes place in probably over 90 per cent of the whole human race,
male and female. The records of art, of language, and the evidence
of actual remains, tend to show that the habitude in question, with
the attendant modification, goes back to very ancient, even perhaps
to palæolithic times.[60] I refer to the preferential use of the
right hand and the enlargement of structure thus brought about in
the right hand and arm. Every right-handed adult man and woman shows
this enlargement of bony and muscular structure. The origin of the
habitude does not concern us here. Let us suppose it due, as Dr. D.
J. Cunningham suggests, to “a transmitted functional pre-eminence
of the left brain,”[61] which is larger than the right, and which
governs the movements of the right side of the body. However this may
be, it is clear that if bodily characteristics acquired by exercise
are transmissible by inheritance the new-born child of right-handed
ancestry ought to show some appreciable preponderance in weight and
size of the right over the left limb. There could hardly be a more
crucial test of the validity of the Lamarckian principle. What do the
investigations of the dissecting-room reveal? I shall quote the two
most recent authorities who have studied this interesting question. Dr.
Cunningham, in the lecture already referred to, writes:—
“Although the matter has not been investigated so fully as to place
the question outside the region of dispute, the evidence at our
disposal distinctly favours the view that at birth the two upper
limbs start upon their individual duties equally endowed in so far
as strength of muscle and size of bones are concerned. Both in mass
and weight the two limbs are to all intents and purposes similar at
birth, and the preponderance in bulk and strength which later on
distinguishes the right arm is acquired during life, and is caused
by the greater amount of work it is called upon to perform.”[62]
Dr. T. G. Moorhead, Chief Demonstrator in Anatomy in Trinity College,
Dublin, after giving the results of the researches of various other
inquirers, writes:—
“From this mass of conflicting evidence I am forced to the
conclusion that no real differences exist.. .. After weighing as
a whole the limbs of eight foetuses I was unable to detect any
constant difference.”[63]
These results appear to conflict most seriously with the theory of the
transmissibility of acquired modifications.
[Illustration: Fig. 3.
_Kallima paralecta_, as it appears at rest, with wings closed.
From Weismann’s The Evolution Theory.
K, the head; B, the limbs.]
Every one is familiar with the fact that species of animals which
are preyed on by others, or which require to be inconspicuous
for the purpose of preying, are very apt to take the colour of
their habitual surroundings. Individuals of the same species will
even differ according to their special habitat. Perhaps the most
marvellous instances of this kind of adaptation are to be found in
certain tropical butterflies, such as the Indian butterfly, _Kallima
paralecta_, here illustrated. We have here, painted on the butterfly’s
wing, the picture of a leaf belonging to a shrub which it frequents—a
picture, when seen under natural conditions, capable of baffling all
but the closest inspection. The different parts—the midrib, the lateral
veinings, the little blotches and spots which represent patches of
mould or drops of water, even the outer contour of the wing itself—all
form an harmonious whole composed of related parts which have
separately no meaning or use. They certainly did not all appear in full
development at the same time. Nor could any one of them, if it appeared
first, have exercised the smallest influence on the appearance of the
others, as the antlers of the elk were supposed to have influenced
the development of the _ligamentum nuchæ_. The early stages must have
been anticipatory of the later ones, but exercise could have had
nothing to do with the result from first to last. The butterfly never
practised looking like a leaf. Nor can any large chemical and elemental
influences have been at work. If nature is capable of producing such
effects as this without the agency of Lamarck’s principle, are there
not excellent grounds for seeking for some other agency which will
cover all the phenomena alike?
Finally let us take the case of the slave-owning ‘Amazon’ ants,
_Polyergus rufescens_. Here we have a case which at the first blush
looks like a perfect picture of an evolutionary process conducted on
the principles of Lamarck’s theory. These ants, it may be supposed,
were originally of the ordinary type of that industrious and
respectable insect, but they were led by the weakness of some of their
neighbours of another species to make occasional attacks on them for
the purpose of carrying off their immature brood, the _pupæ_, as food.
Some of these _pupæ_, near maturity at the period of their capture,
would come out while stored-up in the nest of the conquerors, and when
they did so would immediately set about doing the household work of
the hive as if they were at home. _Polyergus rufescens_ ultimately
became aware that a life of aristocratic leisure awaited him if he
only captured enough _pupæ_ of another species of ant to do his work.
He accordingly confined himself entirely to piratical expeditions of
this nature, and in the course of time underwent a moral and physical
transformation of a most remarkable kind. The ordinary ant instincts
have disappeared in this variety. They do not make their nests, they
do not gather stores, they do not mind their young, they do not even
feed themselves—an Amazon ant will perish of starvation in the presence
of food if there is not a slave ant to put it into his mouth. But
they fight ferociously in their slave-raids, and the form of their
mandible has changed to suit their mode of life. It has become a pair
of sabre-like nippers, excellent for slaying a foe, but ill-adapted
for carrying objects and other industrial occupations. Corresponding
changes have taken place in the head and in the chitinous and muscular
structure.
We have before us, then, what would seem to an uninformed observer,
a striking picture of the acquirement of a certain bodily form and a
certain set of instincts by use, and the total loss of other traits by
disuse, and of the fixing of these characters in a species by heredity.
Yet the picture is altogether an illusion. However we are to explain
the facts—of which more anon—we cannot do so by Lamarckism, for the
simple reason that the peculiar instincts and bodily structure of the
Amazon ants are confined to the so-called ‘worker,’ or in this case
‘soldier,’ caste, which are sexless, and incapable of reproducing their
kind. If these were the individuals which originally started the slave
system among the species, they could not possibly have transmitted the
modifications, moral and physical, which they acquired. The queen-ants,
which normally are the only fertile ants, transmit them, but do not
possess them, and neither do the drones.
The case of these mysterious communities of insects, composed largely
of neuters which do the work of the community but do not reproduce
their kind, was one of the difficulties in the way of Darwin’s theory
of evolution which, he said, staggered him every time he reflected
on it.[64] It is not surprising, therefore, that this difficulty
came to be the battlefield, or a main position thereof, in a most
interesting and illuminating controversy on Natural Selection _versus_
Lamarckism, waged between Mr. Herbert Spencer and Dr. Weismann in the
years 1893-4.[65] Spencer considered the inheritance of acquired
characteristics a factor in evolution of the very first importance;
and so, indeed, from his point of view it is. “Either,” he declared,
“there has been inheritance of acquired characteristics, or there has
been no evolution.” Met by the case, among others, of the slave-making
ants, his explanation is substantially as follows: It was not the
workers (soldiers) which originally acquired military traits, but the
queens, the fully developed females, which lost them. There was once,
as every one admits, a time when all ants, bees, etc. were sexually
mature. There were only males and females. At this stage, possibly, the
Amazon ants were already predatory. It was _then_ that they may have
acquired the military habits and structure, which they were then able
to perpetuate by inheritance.
How, then, did the queens lose these traits? “From the queens,” replies
Spencer, “they have slowly disappeared by inheritance of the effects
of disuse.” The obvious and unanswerable rejoinder made by Weismann
and his followers was that Spencer had only shifted the difficulty
to another ground—from the workers to the queens. If the queens (and
drones) lost the military characteristics by disuse, how do they come
to transmit them unimpaired to the workers? It is the very essence of
Lamarckism that whatever modifications are produced by use or by disuse
shall be transmissible by inheritance.
In this controversy, however, there was another string to the
Lamarckian bow. Worker-ants, bees, etc. are imperfectly developed
females. They have four or five egg-tubes where the queen has two
hundred, but they cannot be fertilized by the drones. It occasionally
happens, however, that these neuter insects do lay a few eggs. These
unfertilized eggs always develop into drones. One of these drones
might, it was suggested, now and then fertilize a genuine queen, and
thus hand on the traits of the worker from which it sprang. But apart
from the fact that an occasional occurrence of this sort would hardly
suffice to maintain the worker-characteristics unimpaired throughout
the ages, there is the decisive answer, as Weismann points out, that
we know at least one species of ant in which the evolution of a
neuter caste is absolutely complete, for the workers of _Tetramorium
caespitum_ possess no egg-tubes at all. Yet the transmission of
characteristics from queens and drones who never exercise them to
workers who cannot pass them on, goes forward in this species of any
ant just as in any other.
Nature, therefore, while doing in the case of these insect communities
exactly what she appears to be doing elsewhere by the accumulation
of acquired characteristics, must, in reality, have been working on
entirely different lines. If we can discover what those lines were,
they will cover the apparently Lamarckian cases as well, but the
Lamarckian principle certainly will not cover these.
In the next chapter we shall review the alternative explanation offered
by Darwinism, the explanation of Weismann; and we shall see whether
Spencer was not as successful in demolishing it as Weismann was in
showing that, if evolution exists at all, some other basis must be
found for it than that on which it was so largely rested by Herbert
Spencer.