of the germ-plasm, the whole quantity of the germ-plasm, or the reduced
size of the germ-cell, may bear an internal relation to the smallness
of the animal which develops therefrom, but it surely cannot be
regarded as absurd to suppose that there is some such relation. There
are no experiments known to me which prove that meagre diet brings
about a progressive decrease in the size of the body. Carl von Voit
has observed that dogs of the same litter grew to very different sizes
of body according as they received abundant or scanty food, but it
would be difficult to make animals small through scantiness of food
and at the same time to keep them capable of reproduction, and thus
proofs of the inheritance of the dwarfing are lacking. Moreover, the
experiments which Nature herself has made are never quite convincing,
because we never can definitely exclude the indirect effect of altered
circumstances. The case of the feral horses of the Falkland Islands, so
often cited since the time of Darwin, which have become small 'through
the damp climate and scanty food,' seems to me, of all known cases of
the kind, the one we should most readily attribute to the direct effect
of continued scanty diet; but even here we cannot altogether exclude
the possibility of the co-operation of adaptations of some kind to the
very peculiar conditions of life in these islands, as far as the feral
horses are concerned. I have not been able to find any record of more
modern exact investigations either regarding these feral horses, or in
regard to the others which are reared in the Falklands under conditions
of domestication. Darwin himself, however, in the Journal of his famous
voyage tells us much that is interesting in regard to the mammals of
the Falkland Islands. Cattle and horses were brought there in 1764 by
the French, and have increased greatly in numbers since that time; they
roam about wild in large herds, and the cattle are strikingly large
and strong, while the horses both wild and tame are rather small, and
have lost so much of their original strength that they cannot be used
for catching wild cattle with the lasso, and horses have to be imported
from La Plata for this purpose. From this contrast between the horses
and the cattle we may at least conclude that it cannot be 'scanty food'
alone which causes the horses to become smaller, but that the climatic
conditions as a whole are concerned in the matter. Whether the total
amount of variation which has taken place in the horses which have
lived wild there for a hundred years would take place in the course of
a single life, or whether it is a cumulative phenomenon, has still to
be decided.
Similar statements, for the most part still more uncertain, are made
in regard to changes in the hair of goats, sheep, cattle, cats, and
sheep-dogs, which are referred to climatic influence. The raw climate
of many highlands, like Tibet and Angora, is said to have directly
produced the long and fine-haired breeds. But there is a lack of proof
that adaptation or artificial selection did not also play a part, and
the fact that similar long-haired breeds have arisen among rabbits
and guinea-pigs in quite different places and under quite different
climatic conditions, but under the directing care of man, speaks in
favour of our supposition. But, on the other hand, it does not seem
impossible that the climate may have a variational influence upon
certain determinants of the germ-plasm, for we have already seen
that the influence of cultivation may incite plants and animals to
hereditary variations, and that slowly increasing disturbances in
the equilibrium of the determinant system may thereby be produced,
which may suddenly find marked expression as 'mutations.' But there
is little probability that _adaptations_, that is, transformations
corresponding to the altered climate, can arise in this way. The thick
fur of the Arctic mammals is assuredly not a direct effect of the cold,
although it has developed in all Arctic animals, not only in the modern
polar bears, foxes, and hares of the polar regions, but also in the
shaggy-haired mammoth of diluvial Siberia, whose tropical relatives of
to-day, the elephants, have an almost naked skin. Another interesting
case, recently brought to light, shows that a group of animals
which, in correspondence with their otherwise exclusively tropical
distribution, have only a moderately developed coat of hair, may, on
migrating to a cold country, grow as good a fur as the members of other
families. I refer to one of the higher apes, _Rhinopithecus roxellanæ_,
which live in companies in the forest on the high mountains of Tibet,
notwithstanding that the snow lies there for six months[26].
[26] See Milne-Edwards, _Recherches pour servir à l'histoire nat. d.
mammifères_, Paris, 1868-74.
But we should assuredly make a mistake if we were to regard the thick
fur of these apes as a direct reaction of the organism to the cold. We
see at once that this cannot be the case if we compare them with marine
mammals, which differ just as much from one another in this respect
and yet are exposed to the same low temperature. The whale and the
dolphin are quite naked, absolutely hairless, but the seals possess a
thick hairy coat. This striking difference is obviously connected with
the mode of life; the whales remain always in the water, the seals
leave it often and therefore require the hairy coat, especially in
colder climates, since otherwise they would be too rapidly cooled by
the evaporation of the water from their bodies. For the whales, on the
other hand, even a very thick hairy coat would not have sufficed as a
protection against cold, since water is a much better conductor of heat
than air, and so it was necessary for them to become enveloped with
the well-known thick layer of blubber, a deposit of fat lying under
the skin, and this--after it was once developed--made the hairy coat
superfluous, so that it disappeared. The seals certainly also possess
a layer of fat under the skin, but it is only in the largest of them
that it affords sufficient protection against the cooling effect of
evaporation when they go upon land or on the ice, and it is therefore
only in these larger ones that the hairy coat has markedly degenerated,
as, for instance, in the walrus and the sea-lion; in all the smaller
seals, in which the mass of the body is much less, the hairy coat is
necessarily very thick and protected from soaking by being very oily,
because the layer of fat under the skin would not be sufficient to
prevent excessive cooling when on land. But the thick coat of hair is
no more _produced_ by the cold than is the layer of fat. As Kükenthal
has shown, all these characters are adaptations, and may depend here as
elsewhere upon natural selection and upon the 'fluctuating' variations
of the germ-plasm upon which that process is based. They are directed
by personal selection because there is the need for them, and they are
produced and augmented by germinal selection.
In all these cases the _direct_ effect of external influences has
nothing to do with the matter, but in other cases that alone brings
about the whole change, which is then limited to the individual and
does not affect the species as a whole at all.
Plant-galls afford striking illustration of the extraordinary changes
that may be brought about in an organism or in its parts by external
influence in the course of the individual life. All possibility of
adaptation on the part of the plant is excluded in this case. The
gall can only depend upon the direct influence of a stimulus, which
is exercised by the young animal, the larva, upon the cells which
surround it; and yet these cells vary to a considerable extent, become
filled with starch or form a woody layer, secrete special substances,
such as tannic acid, in large quantities, or develop hairs, moss-like
growths, pigments, and so on, which do not otherwise occur in that
particular part of the plant. Since Adler and Beyerinck have proved
that it is not a poison conveyed by the mother animal into the leaf or
bud when laying the eggs, which gives rise to the gall-formation, the
matter has become rather clearer. We can now understand that different
stimuli in succession affect the cells which enclose the larva, and
that the ordered succession of these and the exactly graded stimulation
incite the cells to activity in various ways, whether to mere growth
and multiplication in a given direction, or to the secretion of tannic
acid, or to the formation of wood, or to the deposition of reserve
material, and so on. Even the feeble movements of the young larva may
form a stimulus that increases with its growth; then the movements
made by the larva in feeding, and not least the different secretions
emanating from the salivary glands of the animal, which must contain
some substances capable of acting as stimuli and probably changing
in character as time goes on. All these factors must act as specific
stimuli to the plant-cells, influencing and modifying their processes
of growth and metabolism in one direction or another. In principle
at least, if not in detail, we understand the possibility that
through the ordered succession and exact balancing of these different
cell-stimuli the really marvellous structure of the gall may be brought
about as the product of the direct influence, exercised only once,
of the gall-insect upon the plant's parts. But the animal's power of
exercising such a succession of finely graded stimuli upon the plant
must be referred to long-continued processes of selection, and the
structure of the gall, which is adapted to its purpose down to the
minutest details, can thus be understood. The assumption of substances
which can act even in minute quantities as specific cell-stimuli, which
we require to make in this attempt to explain galls, is no longer
without corroboration since we find analogies in the Iodothyrin of
Baumann, the specific secretions of the thymus and the supra-renal
bodies in the higher animals, not to speak of the 'anti-toxins' of the
pathogenic bacteria, which are only known by their effects.
The case of plant-galls is thus of great theoretical interest because
we can exclude all preparation of the plant-cells for the stimuli
exercised by the animal, since the gall is quite useless for the
plant, though many have endeavoured to discover some utility. We
have therefore here a clear case of modification due to the effect,
exercised once only, of external influences, an adaptation of the
animal to the mode of reaction of particular plant-tissues.
It might be supposed that if any inheritance of somatogenic
modifications, any transmission of the acquirements of the personal
part to the germinal part, were possible at all, it would occur in this
case, for many species of gall-insects attack plants, particularly
oaks, in great numbers every year. It has actually been maintained
that galls may arise spontaneously, that is without the presence of a
gall-insect. But no proof of this has ever been found, and the fact
that no one has paid any attention to the assertion probably implies an
unconscious condemnation of the hypothesis of the transmissibility of
acquired characters.
It has been proved by Nägeli's often discussed experiments on hawkweeds
(_Hieracium_) that much less specialized external influences can
give rise to changes which are not hereditary. The Alpine species of
hawkweed varied considerably in their whole habit in the rich soil of
the Botanic Gardens at Munich, but their descendants, when transferred
to a poor flinty soil, returned to the habit of the Alpine species.
The changes which occurred in garden soil were therefore somatic and,
as I have called them, 'transient,' and they did not depend upon
variations of the germ-plasm. It may be objected in regard to these
experiments that they were not continued long enough to prove that
hereditary variations would not also have cropped up in consequence of
the altered conditions. But in any case they prove that marked changes
in the whole body of the plant may occur without any obvious variation
of the germ-plasm. This does not mean, however, that the possibility of
variations of the germ-plasm through such direct external influences
is disputed. We must assume the occurrence of these on _a priori_
grounds, if we refer--as we have done--individual hereditary variation
to fluctuations in the nutrition of the individual determinants of the
germ-plasm. It is probable that many general nutritive variations or
climatic factors affect the germ-plasm as well as the soma, and it is
by no means inconceivable that it is not all, but only certain definite
determinants that are caused to vary.
A proof of this may be found in the results of experiments made upon
the little red-gold fire-butterfly (_Polyommatus phlæas_), to which
I have briefly referred in a former lecture. This little diurnal
butterfly of the family Lycænidæ has a wide distribution and occurs
in two climatic varieties. In the far north and also in the whole of
Germany the upper surface is red-gold with a narrow black outer margin,
but in the south of Europe the red-gold has been almost crowded out by
the black. I reared caterpillars in Germany from eggs of _P. phlæas_
found at Naples and exposed them directly after they had entered on
the pupa-stage to a relatively low temperature (10° C.). Butterflies
emerged which were not quite so black as those of Naples, but
considerably darker than the German form. Conversely, German pupæ were
exposed to greater warmth (38° C.), and these gave rise to butterflies
which were rather less fiery gold and considerably blacker than the
ordinary German form. If I had to repeat these experiments I should
use a much lower temperature in the case of the cold experiments,
because we now know from the experiments of Standfuss, E. Fischer, and
Bachmetjeff, that most of the pupæ of diurnal butterflies can stand a
temperature below zero for a considerable time; probably the results
would be even more marked then.
But even from the results of my former experiments we are justified
in concluding that the blackening of the upper surface of the wing is
really the direct result of the increased temperature during pupahood,
and that the pure red-gold results from the lowered temperature.
Similar experiments made by Merrifield with English _Phlæas_ pupæ agree
exactly with mine. But we may conclude further from these experiments
that both warmth and cold only give rise to slight variations in the
individual pupæ, and that the pure red-gold of the northern form
and the black of the southern are the result of a long process of
inheritance and accumulation, in which the germ-plasm has been caused
to vary in as far as the relevant determinants are concerned, so that
these yield the respective northern and southern forms even in less
extreme temperatures.
As it is to be assumed that these determinants are present not
only in the primordium of the wing in the pupa, but also in the
germ-cells, both must be affected by the varying temperature, and, in
accordance with the continuity of the germ-plasm, each variation of
these determinants, however slight, would be continued in the next
generation. It is thus intelligible that somatic variations like the
blackening of the wings through warmth appear to be directly inherited
and accumulate in the course of generations; in reality, however,
it is not the somatic change itself which is transmitted, but the
corresponding variation evoked by the same external influence in the
relevant determinants of the germ-plasm within the germ-cells, in other
words, in the determinants of the following generation.
This interpretation of these experiments, which I offered some years
ago, has been confirmed in several ways in regard to various other
diurnal Lepidoptera. By employing a temperature as low as 8° C. in
the case of fresh pupæ of various species of _Vanessa_ Standfuss
and Merrifield, and especially E. Fischer, succeeded in getting
great deviations in the marking and colour of the full-grown
insects,--so-called aberrations, such as had previously been found
only very rarely and singly under natural conditions. The deviations
from the normal must undoubtedly be ascribed to the effect of cold,
but it does not follow that they are new forms which have suddenly
sprung into existence, as many have assumed without further experiment.
Dixey, on the other hand, has attempted to establish, by a comparison
of the different species of _Vanessa_, the phyletic development of
their markings, and has found that these aberrations due to cold
are more or less complete reversions to earlier phyletic stages. As
regards the common small painted lady (_Vanessa cardui_), the small
tortoise-shell butterfly (_Vanessa urticæ_), the 'Admiral' (_Vanessa
atalanta_), the peacock (_Vanessa io_), and the large tortoise-shell
(_Vanessa polychioros_), I can agree with this interpretation, and I
do so the more readily because some years ago I suggested that the
alternation of differently coloured generations of seasonally dimorphic
Lepidoptera might be considered as a reversion. But this by no means
excludes the possibility that other than atavistic aberrations may be
produced by cold or heat. There is nothing against this theoretically.
Yet we must not, without due consideration, compare these abruptly
occurring variations to the sport-varieties of plants which we have
already discussed; there is an important difference between the two
sets of cases. In the Lepidoptera a single interference, lasting only
for a short time, modifies the wing-marking, but in the plant varieties
the visible appearance of the variation is preceded by a long period of
preparatory change within the germ-plasm. This period required for the
external influences to take effect was already recognized by Darwin,
and it has recently been named by De Vries the 'premutation period.'
We may explain these remarkable aberrations theoretically in
the following way: The determinants of the wing-scales in the
wing-primordium of the young pupa are influenced by the cold in
different ways, some kinds of determinants being strengthened by it,
others markedly weakened, even crippled so to speak, and in this way
one colour-area spreads itself out more than is normal on the surface
of the wing, and another less, while a third is suppressed altogether.
That this disturbance of the equilibrium between the determinants
leads usually to the development of a phyletically older marking
pattern leads us to the conclusion that in the germ-plasm of the
modern species of _Vanessa_ a certain number of determinants of the
ancestors must be contained in addition to the modern ones. We might
even inquire whether these were not better able to endure cold than
their modern descendants, since their original possessors, the old
species of the Ice age, were accustomed to greater cold, but this idea
is contradicted by the experiments of E. Fischer, which go to show that
the same aberrations are evoked by abnormally high temperature. That
the old ancestral determinants are present in _different_ numbers in
the germ-plasm of the modern species, I am inclined to infer from the
fact that among a large number of experiments made by me in the course
of several years the aberrations have always occurred in very different
numbers in the different broods, although the greatest care was taken
to have the conditions as nearly alike as possible; absolutely alike,
of course, they never can be.
But it would lead me too far if I were to enter on a detailed
discussion of these cases, which have not yet been fully worked up;
only one thing more need be mentioned, that is, that the aberrations
induced by cold are to a certain extent transmissible. Standfuss
first succeeded in making some aberrant specimens of _Vanessa urticæ_
reproduce, and from their eggs he procured butterflies which showed
a much slighter deviation from the normal, which however was still
so decided that it could not be regarded as due to chance. I myself
succeeded in doing the same, but the deviation in this case was much
slighter. But that these observed cases are rightly referred to the
cold to which their parents had been subjected is proved by other
observations recently published by E. Fischer. These refer to one of
the Bombycidæ (_Arctia caja_), which flies by day, and accordingly
has a gay and very definite marking and coloration. A large number
of pupæ were exposed to cold at 8° C., and some of these resulted in
striking and very dark aberrant forms (Fig. 129, _A_). A pair of these
yielded fertilized eggs; in the progeny, which were reared at a normal
temperature, there were among the much more numerous normal forms a
few (17) which exhibited the aberration of the parents, though to a
considerably less degree (Fig. 129, _B_).
This shows that the cold had affected not only the wing-primordia
of the parental pupæ, but the germ-plasm as well, and at the same
time that this latter variation was less marked than that of the
determinants of the wing-rudiments. This gives rise _to an appearance_
of the transmission of acquired characters.
In the case of many of these cold-aberrations in Lepidoptera the cold
gives rise to variations, but does so not by creating anything new,
but by giving the predominance to primary constituents which have long
been present, but are usually suppressed, and so it is also among the
plants. I have in mind, for instance, the interesting experiments of
Vöchting on the influence of light in the production of flowers in
phanerogams. These showed that the common balsam (_Impatiens noli me
tangere_) produces its familiar open flowers in a strong light, but
in weak light only bears small, closed, so-called 'cleistogamous'
flowers. But it would be utterly erroneous to suppose that the strong
or weak light is the real cause, the _causa materialis_, of these two
forms of flowers: the degree of illumination is merely the stimulus
which provokes one or other of the primary constituents to development,
both kinds being present in the constitution of the plant. As has long
been known, the balsam normally possesses two kinds of flowers, and
the slumbering primary constituents of these are so arranged that the
open flowers develop where there is a prospect of insect visits and
cross-fertilization, that is, in sunny weather or in a strong light,
while closed and inconspicuous flowers adapted for self-fertilization
develop in weak light, that is, in shady places and in concealed parts
of the plant, where insect visits are not to be expected.
[Illustration: FIG. 129. _A_, an aberration of _Arctia caja_, produced
by low temperature. _B_, the most divergent member of its progeny.
After E. Fischer.]
Among plants we find thousands of instances of such reactions of
the organism to external stimulus--reactions which are not of a
primary nature, that is, are not the inevitable consequences of
the plant's constitution, but which depend upon adaptations of the
special constitution of a species or group of species to the specific
conditions of its life. To this category belong all the phenomena of
heliotropism, geotropism, and chemotropism, which have been discovered
by the numerous and excellent observations of the plant physiologists.
That all these are adaptations and secondary reactions to stimuli is
proved by the fact that the same stimuli affect the homologous parts of
different species in very different, and often in opposite ways. For
instance, while the green shoots of most plants turn towards the light,
being positively heliotropic, the climbing shoots of the ivy and the
gourd are negatively heliotropic, which is an adaptation to climbing.
In this case the reason of the difference in the mode of reaction
must lie in the difference of constitution of the cellular substance
of the shoot, and since this may differentiate so very diversely in
its relation to light, the power of reaction which plant substance in
general has to light must not be regarded as a primary character, like
the specific gravity of a metal or the chemical affinities of oxygen
and hydrogen, but as adaptations of the living and varying substance
to the special conditions of life. And the origin of these adaptations
must depend upon processes of selection, and on these alone. This is
just the difference between living and non-living matter,--that the
former is variable to a high degree, the latter is not; it is the
fundamental difference upon which the whole possibility of the origin
of an animate world depends.
Among animals also we must distinguish between the direct effects of
external influence to which the organism is not already adapted, and
those reactions which imply a previously established adjustment to the
stimulus. That is, we must distinguish between primary and secondary
reactions.
For instance, Herbst made artificial sea-water in which the sodium was
partially replaced by lithium, and the eggs of sea-urchins developed
in this artificial sea-water into very divergent larvæ of peculiar
structure. We have here a primary reaction of the organism to changed
conditions of life--not an adaptation, not a prepared reaction.
Accordingly these 'lithium larvæ' eventually perished.
The increasing blackness of _Polyommatus phlæas_, which we have already
discussed, must also be regarded as a primary reaction, but not so
the variations--often misinterpreted--of those species of _Artemia_
which live in the brine-pools of the Crimea, in regard to which
Schmankewitsch showed that, when the amount of salt in the water is
diminished, they undergo certain changes which bring them nearer to
the fresh-water form _Branchipus_, while when the salt is increased
in amount they vary in the contrary direction. Probably these are
adaptations to the periodically changing salinity of their habitat.