individual migrant to virgin territory must become variable. This could
not be better illustrated than by the geographical distribution of
terrestrial snails, which emphasizes the fact that a striking wealth of
endemic species is to be found on all oceanic islands. Moreover, the
fact that the number of these endemic species is greater in proportion
to the distance of the island from the continent, indicates that the
variability sets in more intensively and lasts longer in proportion to
the small number of species which become immigrants in the island, and
in proportion to the number of unoccupied areas which are open to the
descendants of the immigrant species. This is undoubtedly the reason
why the Sandwich Islands do not possess _a single species_ which occurs
elsewhere, and the segregation of the unknown ancestral form into many
species and several (four) sub-genera is also to be interpreted in the
same way. There was probably in this case only one immigrant species,
which found a free field, and adapted itself in its descendants to
all the conditions of snail-life which obtained there, and in doing
so split up into numerous and somewhat markedly divergent forms. But
the number of different forms is much greater than the number of
distinctive habitats, as Gulick indicates and substantiates in detail,
for similar areas, if they are relatively isolated from one another,
are inhabited not by the same forms, but by different though nearly
related varieties, and this depends on the fact that from the species
which was in process of varying a different combination of variations
would be sent out at different periods, and the temporary isolation
would result in the evolution of special local varieties.
But I do not believe that this would continue for all time. I rather
think that these--let us say--representative varieties would diminish
in numbers in the course of a long period. For the isolation of single
valley-slopes or of particular woods is not permanent, individuals are
liable to be carried from one to another in the course of centuries as
they were at the beginning of the colonization of the isolated woods;
forests are cleared or displaced by geological changes, connexions are
formed between places, which were formerly separated, and in the course
of another geological period the number of representative varieties,
and probably even of species, will have diminished considerably,--the
former will have been fused together, the latter in part eliminated.
Even now Gulick speaks regretfully of the decimation of rare local
forms by their chief enemies, the mice.
But even if the number of endemic forms in insular regions diminishes
from the time when they were first fully taken possession of, it
nevertheless remains a very high one, for even now Madeira possesses
104 endemic terrestrial snails, the Philippines have more than the
whole of India, and the Antilles as many as the whole American
continent.
Many naturalists believe that each isolated variety must diverge
further and further from its nearest relatives as time goes on.
Although I entirely admit that this is possible, for I have endeavoured
to show that variational tendencies which have once arisen in the
germ-plasm go on in the same direction until they are brought to a full
stop in some way or other, yet I cannot admit that this must always
be so. The species which has been carried to a strange area need not
always contain particular variational tendencies in its germ-plasm, and
need not in every case be impelled to such variations by the influence
of new conditions. We know species which have made their way into new
regions, and, without varying at all, have held their own with, or
even proved superior to, the species which were already settled there.
Many cases of this kind are known, both among plants and animals;
these have been brought by man, intentionally or by chance, from one
continent to another, and have established themselves and spread over
the new area. I need only recall the evening primrose (_Œnothera
biennis_[28]), whose fatherland is Virginia, but whose beautiful big
yellow blossoms now display themselves beside nearly every river in
Germany, having migrated stream-upwards along the gravelly soil; or
the troublesome weed (_Erigeron canadense_), which is now scarcely
less common in our gardens than in those of Canada; or the sparrow
(_Passer domesticus_), which was introduced into the United States to
destroy the caterpillars, but which preferred instead to plunder the
rich stores of corn, and in consequence of these favourable conditions
increased to such an extent that it has now become a veritable pest,
all imaginable means for its extirpation having been tried--as yet,
however, with no great results.
[28] This was written before the appearance of the researches which
De Vries has made on the variations of _Œnothera_ in Europe. Thus
the illustration may not be quite apposite, for it seems to remain
undetermined whether the 'mutations' which occur in Holland do not also
occasionally appear in America. See end of lecture xxxiii.
In all these cases the migration is certainly of recent date, and it is
quite possible that, when a longer time has elapsed, some variations
will take place in the new home, but in any case these instances
prove that an immigrant species can spread over its new area without
immediately varying.
Similarly, it must be admitted that species which have belonged to
two continents ever since Tertiary times need not have diverged since
that time, and we know, for instance, thirty-two species of nocturnal
Lepidoptera which are common to North America and to Europe and yet
exhibit no differences, while twenty-seven other nocturnal Lepidoptera
are, according to Grote, represented in America by 'vicarious' species,
that is, by species which have varied slightly in one or other of the
two areas, perhaps in both.
To sum up: we must undoubtedly admit that isolation has a considerable
influence in the evolution of species, though only in association
with selection in its various grades and modes, especially germinal
selection, natural selection, and sexual selection. We can say
generally that each grade and mode of selection will more readily lead
to the transformation if it be combined with isolation. Thus germinal
selection may call forth slight divergences in colour and marking,
which will be permanent if the individuals concerned are in an isolated
region. In isolation these variations will increase undisturbed, and
in some circumstances will be intensified by sexual selection, so that
the male sex will vary alone in the first place, though the female
may follow, so that ultimately the whole species will be transformed.
Finally, the most marked effect of isolation is seen when individual
members of a species are transferred to virgin territory which offers
unoccupied areas, suitable not to one particular species alone, but to
many nearly related species, so that the immigrant colony can adapt
itself to all the different possibilities of life, and develop into a
whole circle of species. But we saw that such an aftergrowth of new
forms, whether varieties, species, or even genera, may far exceed the
number of different kinds of localities, if there be relative isolation
between the different groups of immigrants within the insular region,
as happens in the case of slow-moving animals like the terrestrial
snails, or of small singing-birds, to which each island of a little
archipelago is a relatively isolated region (Galapagos).
We may thus fully recognize the importance of local isolation without
regarding the absence of crossing with the members of the species in
the original habitat as the sole cause of species-formation, without
setting 'isolation' in the place of the processes of selection.
These last, taken in the wide sense, always remain the indispensable
basis of all transformations, but they certainly do not operate
only in the form of personal selection, but, wherever indifferent
characters are concerned, in that of germinal selection. Here, too,
we see the possibility of reconciliation with those naturalists who
regard transformations as primarily dependent upon internal forces of
development. The fact is that _all variations depend upon internal
causes_, and their course must be guided by forces which work in an
orderly way. But the actual co-operation of all these forces and
variations is not predetermined, but depends to a certain extent upon
chance, for of the possible modes of evolution the one which gains
the upper hand in the play of forces at the moment is alone followed,
the better are everywhere preferred, from the most minute vital units
of the germ-plasm, up to the struggle between individuals and between
species.
LECTURE XXXIII
ORIGIN OF THE SPECIFIC TYPE
Transition species of Celebes snails, according to Sarasin--Possible
variations in the shell due to nutrition--Natural selection plays a
part--Germinal selection--Temporary transitions between species--The
fresh-water snails of Steinheim--How do sharply-defined species
arise?--Nägeli's Developmental Force--The species a complex of
adaptations--Adaptive differences between species--Adaptive nature of
specific characters--The case of Cetaceans--Of birds--Additional note:
the observations and theories of De Vries.
Our study of the influence which geographical isolation may have in
transforming old and giving rise to new forms of life has led us
naturally to a much more important problem, that of the origin of
species as more or less sharply defined groups of forms, and I wish
to make the transition to this problem by discussing another case of
species-splitting effected in association with, or, as is usually
said, _through_ isolation. The naturalists Paul and Fritz Sarasin,
well known through their excellent studies on many components of
the tropical fauna, have published in their latest work interesting
discoveries in regard to the terrestrial snails of Celebes. These
observations show that on this island a great transformation of snails
has taken place, even since the later Tertiary period. A large number
of new species of snail have arisen on this island since that time,
and this, as the authors show to be probable, in association with
the receding of the sea, that is, with the elevation of the island
further out of the water, and thus with the increase of its surface.
The modern terrestrial snails show chains of forms connected in many
ways so that a series of species is connected by transition forms, and
therefore does not really consist of separate species at all, although
the extremes would seem to be separate species if they were studied
by themselves without taking the transition forms into account. The
state of things is exactly as if a Tertiary snail had spread from any
small area over the whole island, and had been transformed slowly
and in a definite direction in accordance with its distance from its
starting-point. It is thus that we must interpret this discovery;
we have here, beside each other in space, and indeed often disposed
along geographical lines, the individual stages of a phyletic process
of transformation, which has reached different levels at different
places. One of the longest of these chains of forms is that of _Nanina
cincta_, which runs across the island from east to west, and, beginning
with the smallest and most delicate forms, ascends through many
intermediate stages to the giant form _N. limbifera_. Such chains of
forms have been previously recognized; thus Kobelt described one in the
case of the Sicilian land-snails of the genus _Iberus_, and other cases
are recorded in literature, but in all instances they refer to areas
which must be regarded as isolated for the snails, and which have been
colonized from a single starting-point.
We have now to inquire whether and how we can explain the origin of
these chains of forms. The cousins Sarasin tell us how they at first
attempted to refer the differences between the individual links of such
a chain to the diverse influence of the external conditions of life,
but in vain; neither the height above sea-level nor the character of
the soil was sufficient, and natural selection was no more so; 'for
why should a high _Obba_-form twisted like a beehive be either better
or worse equipped for the struggle for existence than a smaller and
flatter one?' It is true that we do not understand why, but this does
not seem to me any reason to doubt that natural selection should be
regarded as one of the causes of the divergence of these species,
for we could not answer the same question in regard to any of the
other structural differences between two species of snail, for the
simple reason that we have far too little knowledge of the biological
value of the parts of a snail. Or could any one tell of what use it
would be to a snail-species to have the horns slightly longer, the
foot somewhat narrower, the radula beset with rather larger or more
numerous teeth? We might indeed imagine many ways in which it might
be of advantage, but we are not in a position to say definitely why,
for instance, longer horns should be better for one species than for
another, and yet we do not believe that the structure of snails is
less well adapted to the life of each species than that of any other
animals. The snail's structure is certainly built up of hundreds and
thousands of adaptations, like that of every other animal species,
but while in many others we can, at least in part, recognize the
adaptations as such, we cannot do so at all in regard to the snail.
Simroth has pointed out that the spiral asymmetrical shell bears a
relation to the one-sided opening of the genital organs, but that only
states the general reason for the coiling of the shell. In studying the
differences in the shell one is apt to think of its external appearance
alone, of the protection which it affords to the soft internal organs
of the easily wounded animal; perhaps also of the distribution of
weight, which must be different in a high tower-like structure and a
low flat spiral; possibly, too, of the varied obstacles and resistance
the snail has to encounter in creeping into clefts and holes, or among
a tangle of plants, according to the form of its shell; but is it not
also conceivable that the form of the shell has been determined by its
contents? As Rudolph Leuckart taught, the snail may be regarded as
composed of two parts, one of which is formed by the head and foot,
the other by the so-called 'visceral sac': the former may be called
the animal half, because it chiefly contains the dominant organs--the
nerve-centres, almost the whole mass of muscle, and the sense-organs;
the latter the vegetative half, since it contains the main mass of the
nutritive and reproductive systems--the stomach and intestine, the
large liver, the heart, the kidneys, the reproductive organs, and so
on. The vegetative half of the animal is always concealed within the
shell; would not therefore any great variation in the size of liver,
stomach, intestine, and so on, bring with it a variation in the size
and form of the shell, as well as in the expansion or contraction of
its coils? And might not such variations become necessary because of
some change in the food-supply? It is only a supposition, but it seems
to me very probable that becoming accustomed to a new diet, less easily
broken up and dissolved and of diminished nutritive value, would cause
modification not only of the radula and jaw-plate, but also of the
stomach and the liver, the intestine and the kidneys, whose activity
is closely associated. The stomach must become more voluminous, the
liver which yields the digestive fluid must become more massive, and
so forth. I will not follow this hypothetical example further, for I
merely wished to recall the fact that the snail shell, to the form
of which no biological significance can be commonly attributed, is
actually a sort of external cast of the visceral sac, and consequently
dependent on the variations to which that is liable in accordance
with the conditions of its life. To give precise proofs for such
processes is certainly not yet possible, for we do not even know with
certainty what the diet of the various species of snail is, much less
the difference between the modes of nutrition in two varieties, or the
nutritive value of the materials used, or the changes in secretion,
absorption, assimilation, and excretion which must be brought about by
these differences. But we can at least see that variations in nutrition
must be enough in themselves to give rise to new adaptations in the
size, constitution, and mutual adaptation of the internal vegetative
organs, and we cannot overlook the possibility that the form and
size of the vegetative half, and therefore the form and size of its
secretion, the shell, may also be caused to vary[29]. The fact that we
cannot recognize, for instance, the beehive shape of an _Obba_ as an
adaptation, is thus no proof that it is not one. But let us assume for
the moment that it is not, and that it cannot be referred to natural
selection any more than the other variations in the Celebes chain of
forms, and we may further admit that they cannot be referred to sexual
selection, still less to some 'inherent principle of perfecting,' not
only because there is no question of perfecting in the matter, but
because such a mystical principle is outside of the scope of natural
history and its principles of interpretation.
[29] That this suggestion was not unjustified is evident from a
recent contribution by Simroth ('Ueber die Raublungenschnecken,'
_Naturwissenschaftliche Wochenschrift_, December 8 and 15, 1901). In
this paper the author, who is an expert as regards the biology of
Gastropods, shows that a change of diet may evoke many kinds of changes
in the structure of the food-canal, which may indirectly compel changes
in the shell. Thus in a small indigenous snail, _Daudebardia_, the
pharynx has grown so enormously in thickness and length in adaptation
to the predatory mode of life, that the head and the anterior part of
the body can no longer be retracted within the shelter of the shell.
For this reason, and also because of the snail's habit of following
earthworms into their burrows, the shell has been shunted far back and
obliquely downwards. It has at the same time markedly changed in its
shape, as may still be verified by comparing the form of the shell in
the young stages with that of the adult.
But that transformations in a definite direction can and must arise
from fresh disturbances in the equilibrium of the determinant system,
that is from germinal selection, we have already shown.
Even if the changes of form with which we are here dealing had really
no biological importance, they might quite well have been brought about
by germinal selection, and only one thing remains obscure, that is,
why the different stages on the path of distribution of a species are
at a different level of evolution and not all at the same level. Why
have not all been transformed? Why have some colonies remained near
the ancestral form, while others have varied only a little, and others
again a great deal? This cannot be explained by any assumption of an
internal power of development, and the explanation can only be found
in germinal selection associated with isolation, since the internal
processes in the germ-plasm can quite well run a different course in
different colonies. Nevertheless I am inclined to infer from these
differences in the individual colonies of these chains of forms, that
natural selection in the accepted sense has also played a part in the
evolution of these snail varieties.
Such series of forms are especially interesting, because they show us
the process of species-formation in its different stages beside each
other in space, and thus simultaneously. They represent, so to speak,
a horizontal branch of the genealogical tree of the species, as the
Sarasins well express it, that is, a series of species arising from
each other, which do not break off, but are all capable of life at
the same time, and so exist simultaneously on different areas; they
are species adapted to different localities, not to different times.
The same is true of the snails of other isolated regions, except that
the chains of forms are usually not simple, but split up into several
chains of forms arising from one ancestral form, and under certain
circumstances each of these may break up into two or more diverging
series. The great number of related species in Madeira, or in the
Sandwich Islands, compels us to this assumption, although the branching
of the genealogical trees can no longer be demonstrated with certainty.
This splitting up of forms into several series on a varied insular
region shows us once more that it is germinal selection alone which
forms the basis of all transformations, and that there is not, as
earlier naturalists, especially the botanists Nägeli and Askenasy,
maintained, any peculiar impelling Force of Development innate in
organisms. If there were such a force, a species would be obliged to
go on continuously in the same direction, exactly like the Sarasins'
chains of forms, but no breaking of the species into one or many forms
could occur. But this breaking up into series is easy to understand
when we take germinal selection into consideration, for the germ-plasm
contains many ids and determinants, and each of these can enter upon
new variations, so that one colony can vary in this direction, another
in that, and a great diversity of forms living in isolation must, or at
least may be the result, as we see in the case of the Sandwich Islands.
Let us delay a moment over the Sarasins' case of the Celebes snails. We
are dealing here with series of forms in regard to which the ordinary
conception of species fails us, for they contain varieties whose
extremes are as far apart as distinct species usually are, which are
not, however, distinct, since they are connected with one another by
one and often by several transition forms. Thus we can only break them
up into two or more 'species' by an arbitrary division at one place or
other. The phenomenon itself is not new to us; we have seen that even
Lamarck and Treviranus made use of similar series of forms, connected
by transition stages, in their attack upon the old theory of creation,
and sought to prove by means of these that the idea of species is an
artificial one, read into nature by man, and not innate in nature, and
that the forms of life were only apparently fixed and sharply defined,
being in reality in process of slow transformation. Such beautiful
and convincing examples as we now possess were not available at that
time, but it might even then be said that it was the easier to make
a new species the fewer examples one had to deal with, and the more
difficult the more numerous these became, because with the number of
individuals, especially if they come from a wide area, the number and
diversity of the divergences increases also, so that in many cases, as
in that of the Celebes snails, it becomes impossible to draw a line
between the different species.