338 NATURE 
[August 12, 1886 
varieties, we have many hundreds of thousands of in- 
stances of fertility within the varietal forms with sterility 
towards allied forms. 
Probably enough has now been said to show that, as a 
matter of fact, the particular kind of variation in the re- 
productive system which is required by the theory of 
physiological selection does occur, firstly, in individuals ; 
secondly, in races ; and thirdly, in species. But the evi- 
dence of physiological selection as an agency in the 
evolution of species is so far only prima facie. That is 
to say, although we have evidence to prove the occurrence 
of this particular kind of variation, and although we can 
see that whenever it does occur it must be preserved, as 
yet we have seen no evidence to show how far this kind 
of variation has been at work in the formation of species. 
I will, therefore, next proceed to give an outline sketch of 
the evidence which I have been able to find, tending to 
show that the facts of organic nature are such as they 
ought to be, if it is tru> that physiological selection has 
played any considerable part in their causation. And to 
do this I will begin by taking the three cardinal objections 
to the theory of natural selection with which I set out— 
namely, sterility, intercrossing, and inutility. For, as we 
shall see—and this in itself is a suggestive consideration 
—all the facts which here present formidable obstacles to 
the theory of natural selection, when this is regarded only 
as a theory of the origin of species, are not only explained 
by the theory of physiological selection, but furnish to 
that theory some of the best evidence which I have been 
able to find. 
Argument from Sterility.—In what respects do species 
differ from one another? They differ firstly, chiefly, and 
most generally in respect of their reproductive systems : 
this, therefore, I will call the primary difference. Next, 
they differ in an endless variety of more or less minute 
details of structure, which are sometimes adaptive and 
sometimes not. These, therefore, I will call the secondary 
differences. Now, the secondary differences are never 
numerous as between any two allied species: in almost 
all cases they admit of being represented by units. Yet, 
if it were possible to enumerate all the specific differences 
throughout both the vegetable and animal kingdoms, 
there would be required a row of figures expressive of 
many millions. In other words, the secondary specific 
distinctions may occur in any parts of organisms, but 
never occur in many parts of the same organism. So that, 
if we have regard to the whole range of species, the 
secondary distinctions are seen to be, in the highest 
imaginable degree, variable or inconstant: the only dis- 
tinction which is at all constant or general is the primary 
distinction, or the one which belongs exclusively to the 
reproductive system. Surely, therefore, what we pri- 
marily require in any theory of the origin of sfeczes, 
is an explanation of this relatively constant or general 
distinction. But this is just what all previous theories 
fail to supply. Natural selection accounts for some 
among the many secondary distinctions, but is con- 
fessedly unable to explain the primary distinction. The 
same remark applies to sexual selection, use and disuse, 
economy of growth, correlated variability, and so forth. 
Even the prevention of intercrossing by geographical 
barriers or by migration is unable to explain the very 
general occurrence of some degree of sterility between 
two allied varieties which have diverged sufficiently to take 
rank as different species. All these theories, therefore, are 
here in the same predicament : they profess to be theories 
of the origin of species, and yet none of them is able to 
explain the one fact which more than any other goes to 
constitute the distinction between species and species. 
The consequence is that most evolutionists here fall back 
upon a great assumption: they say it must be the change 
of organisation which causes the sterility—it must be the | 
secondary distinctions which determine the primary. 
But the contrary proposition is surely at least as probable, 
namely, that it is the sterility which, by preventing inter- 
crossing with parent forms, has determined the secondary 
distinctions—or, rather, that it has been the original con- 
dition to the operation of the modifying causes in all cases 
where free intercrossing has not been otherwise prevented. 
For, obviously, it is a pure assumption to say that the 
secondary differences have always been historically prior 
to the primary difference, and that they stand to it in the 
relation of cause to effect. Moreover, the assumption 
does not stand the test of examination, as I will now 
proceed to show. 
(1) On merely a@ friort grounds it scarcely seems 
probable that whenever any part of any organism is 
slightly changed in any way by natural selection, or by 
any other cause, the reproductive system should forthwith 
respond to that change by becoming sterile with allied 
forms. Yet this is really what the assumption in question 
requires, seeing that a// parts of organisms are subject to 
the secondary specific distinctions. What we find in 
nature is a more or less constant association between the 
one primary distinction and an endless profusion of 
secondary distinctions. Now, if this association had been 
between the primary distinction and some one—or even 
some few—secondary distinctions, constantly the same 
| in kind ; in this case I could have seen that the question 
would have been an open one as to which was the condi- 
tional and which the conditioned. But as the case actually 
stands, on merely antecedent grounds, it does not appear 
to me that the question is an open one. Here we havea 
constant peculiarity of the reproductive system, repeated 
over and over again—millions of times—throughout 
organic nature ; and we perpetually find that when this 
peculiar condition of the reproductive system is present, 
it is associated with structural changes elsewhere, which, 
however, may affect any part of any organism, and this in 
any degree. Now, I ask, is it a reasonable view that the 
one constant peculiarity is always the result, and never 
the condition, of any among these millions of inconstant 
and organically minute changes with which it is found 
associated ? 
(2). But, quitting @ prior? grounds, it is a matter of 
notorious fact that in the case of nearly all our innumer- 
able artificial productions, organisms do admit of being 
profoundly changed in a great variety of ways, without 
any reaction on the reproductive system following as a 
consequence. 
(3) Again, as regards wild species, Mr. Darwin proves 
that “ the correspondence between systematic affinity and 
the facility of crossing is by no means strict. A multi- 
tude of cases could be given of very closely allied species 
which will not unite, or only with extreme difficulty ; and, 
on the other hand, of very distinct species which unite 
with the utmost facility.” And he goes on to say that 
“within the limits of the same family, or even of the 
same genus, these opposite cases may occur”; so that 
“the capacity of the species to cross is often completely 
independent of this systematic affinity, that is, of any 
difference in their structure or constitution, excepting 
in their reproductive systems.” Now, on the supposition 
that sterility between species is always, or generally, 
caused by the indirect influence on the reproductive sys- 
tem of changes taking place in other parts of the organ- 
ism, these facts are unintelligible—being, indeed, as a 
mere matter of logic, contradictory of the supposition. 
(4) Mr. Darwin further shows that, ‘independently of 
the question of fertility, in all other respects there is the 
closest general resemblance between hybrids and mon- 
grels.” Clearly, this fact implies that natural selection 
and artificial selection run perfectly parallel in all other 
respects, save in the one respect of reacting on the repro- 
ductive system, where, according to the views against 
which I am arguing, they must be regarded as differing, 
not only constantly, but also profoundly. 
(5) Lastly, Mr. Darwin concedes—or rather insists— 
