August 19, 1886 | 
NATURE 363 
“from the simple circumstance that the more diversified 
the descendants from any one species become in struc- 
ture, constitution, and habits, by so much will they be 
better enabled to seize on many and widely diversified 
places in the economy of nature, and so be enabled to 
increase in numbers.”! And he proceeds to illustrate this 
principle by means of a diagram, showing the hypo- 
thetical divergence of character undergone by the de- 
scendants of seven species. Thus, he attributes divergence 
of character exclusively to the influence of natural selec- 
tion. 
Now, this argument appears to me unassailable in all 
save one particular; but this is a most important parti- 
cular: the argument wholly ignores the effect of inter- 
crossing with parent forms. Granting to the argument 
that intercrossing with parent forms is prohibited, and 
nothing can be more satisfactory. The argument, how- 
ever, sets out with showing that it is in limited areas, or 
in areas already overstocked with the specific forms in 
question, that the advantages to be derived from diversi- 
fication will be most pronounced. Or, in Mr. Darwin’s 
words, it is where they “ jostle each other most closely” 
that natural selection will set a premium upon any 
members of the species which may depart from the 
common type. Now, inasmuch as this jostling or over- 
crowding of individuals is a needful condition to the 
agency of natural selection in the way of diversifying 
character, must we not feel that the general difficulty 
from intercrossing previously considered is here pre- 
sented in a special and aggravated form? At all events, 
I know that, after having duly and impartially considered 
the matter, to me it does appear that, unless the swamping 
effects of intercrossing with the parent form on an over- 
crowded area is in some way prevented to begin with, 
natural selection could never have any material supplied by 
which to go on with. Let it be observed that I regard 
Mr. Darwin’s argument as perfectly sound where it treats 
of the divergence of sfeczes from one another—ze. of the 
rise of genera, families, &c.; for then physiological barriers 
are present to prevent intercrossing. But in applying 
the argument to explain the divergence of znazviduals 
into varieties it seems to me that here, more than any- 
where else, he has lost sight of the formidable difficulty in 
question. For in this particular case so formidable does 
the difficulty seem to me, that I cannot believe natural 
selection alone could produce any divergence of character 
so long as all the individuals on an overcrowded area 
occupy that area together. Yet if any of them quit that 
area, and so escape from the unifying influence of free 
intercrossing, these individuals also escape from the con- 
ditions which Mr. Darwin names as those that are needed 
by natural selection in order to produce divergence. 
Therefore it appears to me that, under the circumstances 
supposed, natural selection alone could not produce 
divergence ; the most it could do would be to change the 
whole specific type in some one direction (the needful 
variations in that one direction being caused by some 
general change of food, climate, habit, &c., affecting a 
number of individuals simultaneously), and thus induce 
transmutation of species in a linear series—each succeed- 
ing member of which might supplant its parent form. 
But, in order to secure diversity. multiplication, or rami- 
fication of species, it appears to me obvious that the 
primary condition required is that of preventing inter- 
crossing with parent forms at the origin of each branch— 
whether the prevention be from the first absolute, or only 
partial. And, after all that has been previously said, it is 
needless again to show that the principles of physiological 
selection are at once the only principles which are here 
likely to be efficient, and the principles which are fully 
capable of doing all that is required. For species, as they 
now stand, unquestionably prove the fact of ramification ; 
and it appears to me no less unquestionable that ramifi- 
* “ Origin of Species,” p. 87. 
cation, as often as it has occurred, can only have been 
permitted to occur by the absence of intercrossing with 
parent forms. But apart from geographical barriers 
(which, according to Mr. Darwin’s argument, would be 
inimical to the divergence of character by natural selec- 
tion), the ramification can only take place as a conse- 
quence of physiological selection, or as a consequence of 
some change in the reproductive system which prevents 
intercrossing with unchanged (or differently changed) 
compatriots. But when once this condition is supplied by 
physiological selection, I have no doubt that divergence 
of character may then be promoted by natural selection, 
in the way that is explained by Mr. Darwin. 
From which it will be seen that the theory of physiological 
selection has this advantage over the theory of natural selec- 
tion in the way of explaining what Mr. Darwin calls diversi- 
fication of character, or what I have called the ramification 
of species. This diversification or ramification has refer- 
ence chiefly to the secondary specific distinctions, which, 
as we have seen, the theory of naturai selection supposes 
to be the first changes that occur, and, by their occur- 
rence, to induce the primary distinction of sterility. My 
theory, on the other hand, inverts this order, and supposes 
the primary distinction to be likewise (in most cases) the 
primordial distinction. Now, the advantages thus gained 
are twofold. In the first place, as just shown, we are able 
to release the principle of natural selection from what 
appears to me the otherwise hopeless difficulty of effect- 
ing diversification of character on an overcrowded area 
with nothing to prevent free intercrossing. And, in the 
next place, as we can now see, we are able to find an 
additional reason for the diversification of character, over 
and above the one that is relied upon by Mr. Darwin. 
For, by regarding the primary distinction of sterility as 
likewise the primordial distinction, we are able to apply 
to an incipient variety, inhabiting even an overcrowded 
area, the same principles which are known to lead to 
diversification by geographical barriers or by migration, 
as ‘previously explained. In other words, if once we 
regard the primary distinction of sterility as also the 
initial distinction, instead of the incidental result of 
secondary distinctions, Mr. Darwin’s argument touching 
the causes of diversification is not merely saved : it is 
notably extended by the addition of an independent prin- 
ciple, which, as we know from other evidence, is a principle 
of high importance in this respect. 
Argument from Geographical Distribution.—The body 
of evidence under this head is too large to be given in 
an abstract; but the following are some of the chief 
points. 
Mr. Darwin too’ a great deal of trouble to collect 
evidence on the two following facts, namely, (1) that 
“species of the larger genera in each country vary more 
frequently than the species of smaller genera” ; and (2) 
that “many of the species included within the larger 
genera resemble varieties in being very closely, but un- 
equally, related to each other.” By larger genera he 
means genera containing many species, and he accounts 
for these gereral facts by the principle “that where many 
species of a genus have been formed, on an average many 
are still forming.” But how forming? If we say by 
natural selection alone, we should expect to find the 
multitudinous species differing from one another in 
respect of features presenting utilitarian significance ; 
yet this is precisely what we do not find. For Mr. Dar- 
win’s argument here consists in showing that “in large 
genera the amount of difference between the species is 
often exceedingly small, so that in this respect the species 
of the larger genera resemble varieties more than do the 
species of the smaller genera.” Therefore the argument, 
while undoubtedly a very forcible one in favour of the 
fact of evolution, appears to me scarcely consistent with 
the theory of zatwral selection. On the other hand, the 
argument tells strongly (though unconsciously) in favour 
