definite direction results as inevitably when it is called forth by the
indirect influence of conditions, that is, through the need for a new
adaptation, as when it is induced in the germ-plasm by direct causes
such as those of climate, we shall not be disposed to estimate very
highly the part played by mutual sterility in the origin of species. We
shall rather be inclined to assign it a rôle at a later stage, after
the separation of the forms has taken place, and this view is supported
by the fact of the mutual sterility of most nearly related species, and
by the theoretical consideration that the frequency of hybrids, even if
these are always eliminated in the struggle for existence, must signify
a loss for both the parent species. But no certain conclusion can be
based upon either of these arguments--not upon the theoretical one,
because here again we are unable to estimate the extent of this loss;
and not upon the argument from fact, because the results of experiments
in crossing animals have generally been overestimated, since we are apt
to regard the most nearly related animals that are at our disposal as
being very closely related. Thus, for instance, horse and ass, horse
and zebra are undoubtedly rightly included within the same genus, but
the fact that there are several species of zebra in Africa gives us an
idea of the number of transition stages that may have existed between
the horse and the zebra. Entomologists have sometimes reared hybrids
between the most nearly related indigenous species of hawk-moth of
the genus _Smerinthus_--hybrids of _Smerinthus ocellata_, the eyed
hawk-moth, and _Smerinthus populi_, the poplar hawk-moth. I have myself
made many experiments of this kind, and have often succeeded in getting
the two species to pair and even to deposit eggs, but I have never
seen a caterpillar emerge from them. The hybrids do occur, however,
and they have been repeatedly obtained by Standfuss. In external
appearance they are intermediate between the parent forms, but with
marked divergences, thus, for instance, the beautiful blue eye on the
posterior wing of _S. ocellata_ (Fig. 5, vol. i. p. 69) may have almost
disappeared or be only indicated. They are sterile. But we know three
species of _Smerinthus_ in North America, which are all much nearer to
_S. ocellata_ than _S. populi_ is, for they all possess the eye-spot
referred to, although it is less well developed. The proof that the
most nearly related species do not yield fertile descendants should be
sought for by crossing _Smerinthus ocellata_ with one of these American
species if it is to have any decisive value.
Experiments of the same kind have been made by Standfuss with different
species of indigenous _Saturnia_, and these have shown not only that
crossing is possible, but that the hybrids are fertile in their turn.
These results are to be valued the more highly because it is well known
that Lepidoptera, and even the usually prolific silk-moths, do not
readily reproduce in captivity, even within the same species. We have
in _Saturnia pyri_, _spini_, and _carpini_ three well-marked distinct
species with no intermediate forms in nature, and with quite different
colouring in the caterpillars. That these should have been successfully
combined in a triple hybrid proves at least that sexual alienation
cannot have advanced far in this case.
We must beware, however, of attributing too much to the constant
mutual crossing which occurs in a species living on a connected area
and of regarding its influence as irresistible. Undoubtedly it must
go far towards securing the uniformity of individuals, but not only
is it unable to achieve this, but it cannot successfully resist the
stronger influences making for variation which may be exerted upon a
part of the area of the species. We have already seen that it is quite
erroneous to suppose that every new adaptation must be lost sight of
again because of the continual crossing with other members of the
species upon the same area. Other things being equal, this depends
entirely upon the importance of the adaptation in question. Just as
climatic influences may be so strong that they entirely overcome the
influence of crossing, and give rise to a local race notwithstanding
imperfect geographical isolation, so the same may happen in the case of
adaptations. It is quite conceivable that the polar hare of Scandinavia
may have evolved a whole series of races, each of which is adapted
to the duration of the snow in its geographical range, although a
crossing of these quick-footed animals must frequently occur in the
course of time, even as regards forms from widely separated areas, and
although the whole region is inhabited without a break by the species,
so that a 'mingling' of the hares of all regions from south to north,
and conversely, may take place, and indeed must be continually taking
place, though of course very slowly.
It is precisely this extreme slowness with which the intermingling
of racial characters take place that seems to me essential for the
production of local or, as in this case, regional races. It is not
difficult to calculate the rate of 'blood-distribution' if we assume
that the conditions for a rapid dissemination are as favourable as
possible. Let us assume that it takes place along a certain line--in
this case from south to north--and that the numerical strength of
the species remains constant, each pair of hares yielding a pair of
surviving offspring, which will attain to reproduction. Let us suppose
that one of these hares moves his home northwards to the extent of his
range, that is, as far as a hare is accustomed to range from his head
quarters, and that he pairs with one of the descendants on the next
stretch.
Let us further suppose that this stretch is ten kilometres in extent,
and that the change of quarters take place once in each year, then the
blood of a South Scandinavian hare would have extended ten kilometres
further north in ten years, and in a hundred years 100 kilometres; it
would not, however, be quite pure, but mixed and thinned by crossing
with a hundred mates of different individual bloods, that is, thinned
to the extent of 2 to the 100^{th} power, that is, to less than a
millionth part. Thus even with these much too favourable assumptions
the influence of a region of hares 100 kilometres distant would be
actually _nil_ upon the inhabitants of a region which was in process
of new adaptation. That the assumptions are too favourable is quite
obvious, since every surviving hare would not be likely to move his
home, and probably the majority would remain in the old quarters
and find mates there. The blood-mingling would therefore take place
much more rarely, perhaps only once in ten years, and the wandering
descendants of the second generation might move southward, and so
neutralize the previous blood-mingling, and so on. But let us keep
to our favourable assumptions, and attempt to determine how strong
the assimilating influence of the blood-mingling from south to north
would be upon a point _A_. The blood of the nearest stretch diluted
to a half would affect the inhabitants of _A_ once in each year; the
second stretch would only contribute blood of 1/4 strength, the third
of 1/8, the fourth of 1/16, and the blood of the tenth would be diluted
to 1/1024. A region _B_, extending over twenty such stretches, or 200
kilometres, would thus shelter within it a hare population of which
the centre would only be influenced from the periphery in vanishing
proportions. If the winter were of equal length over the whole area of
_B_, all the inhabitants would be tending to vary the period for which
the winter dress was worn in correspondence with the length of the
winter, and the centre of the region would be the less impeded in this
process because the more peripheral areas would also be approximating
to the same adaptation. But since even the admixture of 1/32 of strange
blood could have no hindering influence upon a variation, there would
remain a region of 2 × 5 = 10 stretches upon which the influence of
the non-varying regions would be without effect. There would therefore
arise a new race in relation to the duration of the winter dress, and
this would not cease abruptly, but would gradually pass over into the
neighbouring regions, which however would be pure at their centre, just
as is probably the case in reality, if we regard _B_ as any point in
the line of distribution from south to north.
The harmony of the individuals within a species will therefore depend
in part upon the mingling of hereditary primary constituents associated
with reproduction, but in greater part upon adaptation to the same
conditions; it is _a similarity of adaptation_, and the strongest
influence which sexual reproduction exerts lies not in the mingling of
these hereditary constituents alone, but above all in the reduction
in the germ-plasm of the two parental hereditary contributions--a
reduction which results from and through the sexual intermingling. It
is only this that prevents these primary constituents from varying at
too unequal a rate in the transformations of species, and causes them
ultimately to resemble each other closely again.
But while mutual sterility is not an absolutely necessary condition in
the separation of species, it would be going too far in the opposite
direction to regard mutual fertility as something general, or to
attribute to it a rôle in the origination of new species.
Certain botanists, like Kerner von Marilaun, regard the mingling of
species as a means of forming new species with better adaptations;
they suppose that fertile hybrids may, in certain circumstances, crowd
out the parent species, and themselves become new species. It will be
admitted that such cases do occur, that, for instance, in the north of
Europe the hybrid between the large and the small water-lily, _Nuphar
luteum_ and _Nuphar pumilum_, to which the name _Nuphar intermedium_
has been given, has driven both the parent species from the field,
because its seeds mature earlier, and it is therefore better adapted
to the short vegetative period of the north, but nevertheless we must
maintain that the evolution of species on the whole does not take
place through hybridization. Such cases are probably nothing more than
rare exceptions. This is corroborated by the entire insignificance of
hybridization in animals, among which species appear in the same way as
they do in plants, and where the mingling of two species occurs only
sporadically and in a few species, never to any very great extent.
If species are complexes of adaptations, based in each case on the
given physical constitution of the parent species, then we can readily
understand the fact that they are in our experience not fixed or
eternal, but that they change in the course of the earth's history.
The numerous fossil remains in the various strata of the earth's crust
prove that this is true in a high degree, that in almost every one of
the more important geological strata new species occur, and that not
only species and genera, but families, orders, indeed whole classes
of animals, which lived at one time, have now completely disappeared
from the face of the earth. We can understand this phenomenon when we
reflect that the conditions of life have also been slowly changing
through the course of the earth's history, so that the old species had
only the alternative of dying out, or of becoming transformed into new
species.
But simple as this conclusion is, it can hardly be deduced with
certainty from the occurrence and succession of the fossil species
alone. For instance, we should strive in vain to recognize the
cause which led one of those regularly arranged snail-species of
the Steinheim lake basin to become transformed into one or two new
species at a particular time, or to find the cause which moved those
curious tripartite Crustaceans of primitive times, the Trilobites,
which peopled the Silurian seas with such a wealth of forms, to
become suddenly scarce towards the end of the Silurian period, and
to disappear altogether in the succeeding period, the Devonian. The
famous geologist Neumayr sought to refer this striking phenomenon to
the fact that just at that time the Cephalopods, 'the most formidable
and savage marauders among the invertebrate marine fauna,' gained the
ascendancy, and it is quite possible that he was right in his surmise,
but who is to prove it? Can we decide even in the case of animals now
living whether the losses inflicted on a much persecuted species by an
abundant and greedy persecutor exceed the numbers of progeny, and are
therefore driving the species gradually towards extermination? Probable
as such a supposition appears, it cannot be accepted as proven.
Since in many cases of the extinction of great animal-groups we cannot
even prove that there was a simultaneous ascendancy of powerful
enemies, other factors must be discovered to which the apparently
sudden disappearance may be attributed. Many naturalists have tried
to guess at internal reasons for extinction, and have adopted the
theory--associated with the tendency to assume mystical principles of
evolution--that species in dying out are obeying an internal necessity,
as if their birth and death were predestined, as it is in the case of
multicellular individuals, as if there were a _physiological death of
the species_ as there is of the multicellular individual.
Neumayr showed, however, that the facts of palæontology afford no
support for this view. I need not repeat his arguments, but will simply
refer to his clear and concise exposition of the problem. It is obvious
that our theory of the extinction of species as due to external causes
cannot be rejected on the ground that our knowledge of the struggle
that species had to maintain for their existence in past times is
even mere imperfect than our knowledge of the struggle nowadays, and
that we are frequently unable to judge of it at all. But the facts of
geology are of value in another, quite different way. They reveal such
an extraordinary dissimilarity in the duration of species, and also
of the great groups of organisms, that the dissimilarity of itself
is sufficient to prevent our regarding the extinction of species as
regulated by _internal_ causes. Certain genera of Echinoderms, such as
starfish (_Astropecten_), lived in the Silurian times, and they are
represented nowadays in our seas by a number of species: and in the
same way the Cephalopod genus _Nautilus_ has maintained itself among
the living all through the enormous period from the Silurian sea to our
own day. Formerly the Nautilids formed a predatory horde that peopled
the seas, and, as we have seen, we may perhaps attribute to their
dominance the disappearance of an order of Crustaceans, the Trilobites,
which were equally abundant at that period. Now only a single species
of nautilus (_Nautilus pompilius_) lives on the coral reefs of the
southern seas. Similarly, the genus _Lingula_ of the nearly extinct
class of Brachiopods, somewhat mussel-like sessile marine animals, has
been preserved from the grey dawn of primitive times, with its records
in the oldest deposits, and is represented in the living world of
to-day by the so-called 'barnacle-goose' mussel, _Lingula anatina_.
On the other hand, we know of numerous species which lasted for quite
a short time, such as, for instance, the individual members of the
series of Steinheim _Planorbis_ species, or of the Slavonic _Paludina_
species. Not infrequently, too, genera make their appearance and
disappear again within the period of one and the same geological
stratum.
These facts not only tell against an unknown vitalistic principle of
evolution, but in general against the idea of the determination of
the great paths of evolution by purely internal causes. If there were
a principle of evolution the dissimilarity in the duration of life
could not be so excessive; if there were a 'senile stage' of species
and a natural death of species comparable to the natural death of
individuals, it would not have been possible for most of the Nautilidæ
to have been restricted to the Silurian epoch, and yet for one species
to have continued to live till now; and if there were a 'tendency'
of species to vary persistently onwards, and to 'become further and
further removed from the primitive type,' as has been maintained, then
such ancient and primitive Cephalopod forms like the _Nautilus_-species
could not have persisted until now, but must long ago have Wen
transmuted into higher forms. The converse, however, is conceivable
enough, namely, that the great mass of the species of a group such as
the Nautilidæ were crowded out by superior rivals in the struggle for
existence, but that certain species were able to survive on specially
protected or otherwise favoured areas. We have a fine example of this
in the few still living species of the otherwise extinct class of
Ganoid fishes. During the Primary and Secondary epochs these Ganoids
peopled all the seas, but at the boundary between the Cretaceous and
the Tertiary period they retrograded considerably, simultaneously with
the great development of bony fishes or Teleosteans, and now they are
only represented by a dozen species distributed over the earth, and
most of these are purely river forms, while the others at least ascend
the rivers during the spawning season to secure the safety of their
progeny. For the rivers are sheltered areas as compared with the seas,
and large fishes like the Ganoids will be able there to hold their
own in the struggle better than they could in the incomparably more
abundantly peopled sea.
Thus I can only regard it as playing with ideas to speak of birth,
blossoming, standstill, decay, and death of species in any other than a
figurative sense. Undoubtedly the life of the species may be compared
with that of the individual, and if the comparison be used only to make
clear the difference between the causes of the two kinds of phenomena,
there can be no objection to it, only we must beware of thinking we
have explained anything we do not know by comparing it with something
else that is also unknown.
We have already shown that the natural death of multicellular organisms
is a phenomenon which first made its appearance with the separation
of the organism into somatic or body cells and reproductive or germ
cells, and that death is not an inevitable Nemesis of every life, for
unicellular organisms do not necessarily die, though they may be killed
by violence. These unicellular organisms have thus no natural death,
and we have to explain its occurrence among multicellular organisms
as an adaptation to the cellular differentiation, which makes the
unlimited continuance of the life of the whole organism unnecessary and
purposeless, and even prejudicial to the continuance of the species.
For the species it is enough if the germ-cells alone retain the
potential immortality of the unicellulars, while, on the other hand,
the high differentiation of the somatic cells necessarily involves
that they should wear themselves away in the performance of their
functions, and so become subject to death, or at least that they should
undergo such changes that they are no longer capable of functioning
properly, so that thus the organism as a whole loses the power of life.
There can be no doubt whatever that death is virtually implied in
the very constitution of a multicellular organism, and is thus, so
to speak, a foreseen occurrence, the inevitable end of a development
which begins with the egg-cell and reaches its highest point with the
liberation of the germ-cells, that is, with reproduction, and then
enters on a longer or shorter period of decadence, leading to the
natural death of the individual.
It is only by straining the analogy that this course of development
can be compared with the origin and transformation or extinction of
species. Not even the entirely external analogy of the blossoming
from a small beginning and the subsequent decay is always correct;
for in the fresh-water snails of Steinheim, at any rate, almost the
whole of the members of the species underwent a transformation at a
particular time, and became a new species, which was after a long
time retransformed without any appreciable decrease in the number of
individuals being observable. To speak of a 'senile stage' of the
species, of a stiffening of its form, of an incapacity for further
transformation, is to indulge in a play of fancy quite inadmissible in
the domain of natural science.
It is admitted, however, that there is a correct idea at the base of
all this, for many species have not passed over into new forms, but
have simply died out because they were unable to adapt themselves
to changed conditions. This did not happen because they had become
incapable of variation, but because they could not produce variations
of sufficient magnitude, or variations of the kind required to enable
the species to remain an active competitor in the struggle for
existence.
It obviously depends upon the coincidence of manifold circumstances,
whether an adaptation can be successfully effected or not. Above all,
it must be able to keep pace with the changes in the conditions of
life, for if these advance at a more rapid rate the organisms will
succumb in the midst of the attempt at adaptation. It is probably
in this way that the striking disappearance of the Trilobites is to
be explained, as Neumayr has pointed out, for the Nautilidæ, a new
group of enemies, multiplied so quickly at their expense that they
had not time to evolve any effective means of protection. It cannot
be maintained for a moment that every species is able to protect
itself against extermination by any other; the increased fertility,
the increased rapidity of locomotion, the increased intelligence
and similar qualities, may all be insufficient, and then extinction
follows; not, however, because the species has become 'senile,' but
because the variations possible to its organization do not suffice to
maintain it in the struggle.