August 5, 1886] 
point, since, as I have already observed, its force has been 
fully recognised by Mr. Darwin and his followers. Here 
again, therefore, the theory of natural selection fails as a 
theory of the origin of species.! 
In view of these three grave disabilities under which 
the theory of natural selection lies, I feel entitled to affirm 
that the theory has been misnamed. Natural selection is 
not, properly speaking, a theory of the origin of sfeczes : 
it is a theory of the origin—or rather of the cumulative 
development—of adaptations, whether these be morpho- 
logical, physiological, or psychological ; and whether they 
occur in species only, or likewise in genera, families, 
orders, or classes. These two things are very far from 
being the same; for, on the one hand, in an enormously 
preponderating number of instances, adaptive structures 
are common to numerous species, while, on the other 
hand, the features which serve to distinguish species from 
species are, as we have just seen, by no means invariably 
—or even generally—of any adaptive character. If once 
it is thus clearly perceived that the theory of natural 
selection is not a theory of the origin of species, but a 
theory of the development of adaptive structures—whether 
these happen to be distinctive of species or of higher taxo- 
nomical divisions—if once this is clearly perceived, the 
theory is released from all the difficulties which we have 
been considering. For these difficulties have beset the 
theory only because it has been made to pose as a theory 
of the origin of species, whereas in point of fact it is 
nothing of the kind. In so far as natural selection has 
had anything to do with the genesis of species, its opera- 
tion has been, so to speak, incidental: it has only helped 
in the work of originating species in so faras some among 
the adaptive variations which it has preserved happen to 
have constituted differences of merely specific value. 
Many other such differences there are with which natural 
selection has had nothing to do—particularly the most 
universal of all such differences, or that of mutual sterility 
—while, on the other hand, by far the larger number of 
adaptations which have been the work of natural selection 
are now the common property of genera, families, orders, 
or classes. Let it, therefore, be clearly understood that it 
is the office of natural selection to evolve adaptations : 
not necessarily, or even generally, to originate species. 
Let it also be clearly understood that in thus seeking to 
place the theory of natural selection on its true logical 
footing, I am in no wise detracting from the importance” 
of that theory. On the contrary, I am but seeking to 
release the theory from the difficulties with which it has 
been hitherto illegitimately surrounded. 
Enough has now been said to justify the view that there 
must be some cause or causes other than natural selection 
operating in the evolution of species. And this is no 
more than Mr. Darwin himself has expressly and repeat- 
edly stated to have been his own view of the matter ; nor 
am I aware that any of his followers have thought other- 
wise. Hitherto the only additional causes of any import- 
ance that have been assigned are use and disuse, sexual 
selection, correlated variability, and yet another principle 
which I believe to have been of much more importance 
than any of these. Yet it has attracted so little attention 
as scarcely ever to be noticed by writers on evolution, and 
never even to have received a name. For the sake of 
convenience, therefore, I will call this principle the Pre- 
vention of Intercrossing with Parent Forms, or the 
Evolution of Species by Independent Variation. 
First let us consider how enormous must be the number 
of variations presented by every generation of every 
© Of the three cardinal objections to the theory of natural selection thus 
briefly stated, Mr. Darwin himself appears to have attributed most import- 
ance to the first, seeing that its consideration occupies so large a portion of 
his writings. The objection from intercrossing, on the other hand (which was 
first rendered with much force and clearness by the late Prof. Fleeming 
Jenkin of Ediuburgh, in ananonymous article, North British Review, 1867), | 
is the only difficulty in the way of his theory which Mr. Darwin can fairly 
be said not to have sufficiently treated. The objection from inutility was first 
prominently raised by Bronn. It was afterwards developed by Nageli, Broca, 
Mivart, and many other writers. 
NATURE 
315 
species. According to the Darwinian theory it is for the 
most part only those variations which happen to have 
been useful that have been preserved: yet, even as thus 
limited, the principle of variability is held able to furnish 
sufficient material out of which to construct the whole 
adaptive morphology of nature. How immense, therefore, 
must be the number of unuseful variations! Yet these are 
all for the most part still-born, or allowed to die out imme- 
diately by intercrossing. Should such intercrossing be pre- 
vented, however, there is no reason why unuseful variations 
should not be perpetuated by heredity quite as well as useful 
ones when under the nursing influence of natural selection 
as, indeed, we see to be the case in our domesticated 
productions. Consequently, if from any reason a section 
of a species is prevented from intercrossing with the rest 
of its species, we might expect that new varieties (for the 
most part of a trivial and unuseful kind) should arise 
within that section, and that in time these varieties should 
pass into new species. And this is exactly what we do 
find. Oceanic islands, for example, are well known to be 
extraordinarily rich in peculiar species ; and this can best 
be explained by considering that a complete separation of 
the fauna and flora of such an island permits them to 
develop independent histories of their own, without inter- 
ference by intercrossing with their originally parent forms. 
We see the same principle exemplified by the influence of 
geographical barriers of any kind, and also by the 
consequences of migration. When a species begins to 
disperse in different directions from its original home, 
those members of it which constitute the vanguard of 
each advancing army are much more likely to perpetuate 
any individual variations that may arise among them 
than are the menbers which still occupy the original 
home. For not only is the population much less dense 
on the outskirts of the area occupied by the advanced 
guard ; but beyond these outskirts there lies a wholly un- 
occupied territory, upon which the new variety may gain 
a footing during the progress of its further migration. 
Thus, instead of being met on all sides by the swamping 
effects of intercrossing with its parent form, the new 
variety is now free to perpetuate itself with comparatively 
little risk of any such immediate extinction. And, in the 
result, wherever we meet with a chain of nearly allied 
specific forms so distributed as to be suggestive of migra- 
tion with continuous modification, the points of specific 
difference are trivial or non-utilitarian in character. 
Clearly this general fact is in itself enough to prove that, 
given an absence of overwhelming intercrossing, indepen- 
dent. variability may be trusted to evolve new species. 
The evidence which I have collected, and am collecting, 
of the general fact in question, must be left to constitute 
the subject of a future paper.t 
Were it not for the very general occurrence of some 
degree of sterility between even closely allied species, and 
were it not also for the fact that closely allied species are 
not always separated from one another by geographical 
barriers, one might reasonably be disposed to attribute all 
cases of species-formation by independent variability to 
the prevention of intercrossing by geographical barriers, 
or by migration. But it is evident that these two facts 
can no more be explained by the influence of geographical 
barriers or by migration than they can by the influence 
of natural selection. The object of the present paper is 
to suggest an additional factor in the formation of specific 
types by independent variability, and one which appears 
to me fully competent to explain both the general facts 
just mentioned. 
t So far as I am aware, the first writer who insisted on the importance of 
the prevention of intercrossing in the evolution of species, both by isolation 
and migration, was Moritz Wagner. Since then Wallace, Weismann, and 
others have recognised this factor. The most recent contribution to the 
subject is an admirable collection of facts published by Mr, Charles Dixon in 
a work entitled, ‘‘ Evolution without Natural Selection,’’ which was recently 
reviewed in thesecolumns. But I cannot find that any of these writers allude 
to the principle which it is the object of the present paper to enunciate, and 
which is explained in the succeeding paragraphs. 
