August 12, 1886] 
that “the primary cause of the sterility of crossed species 
is confined to differences in their sexual elements.” A 
general fact which assuredly proves that the primary 
specific distinction is one with which the organism as a 
whole is not concerned: it is merely a local variation 
which is concerned only with the sexual system. Why, 
then, should we suppose that it differs from a local varia- 
tion taking place in any other part of the organism ? Why 
should we suppose that, unlike all other such variations, 
it can never be independent, but must always be super- 
induced as a secondary result of changes taking place 
elsewhere? It appears to me that the enly reason why 
evolutionists suppose this is because the particular varia- 
tion in question happens to have as its result the origina- 
tion of species ; and that, being already committed to a 
belief in natural selection or other agencies as the causes 
of such origination, they are led to regard this particular 
kind of local variation as not independent, but super- 
induced as a secondary result of these other agencies 
operating on other parts of the organism. But once let 
evolutionists clearly perceive that natural selection is con- 
cerned with the origin of species only in so far as it is 
concerned with the origin of adaptive structures—or of 
some among the secondary distinctions—and they will 
perceive that the primary specific distinction takes its 
place beside all other variations as a variation of a local 
character, which may, indeed, at times be due to the 
indirect influence of natural selection, use, disuse, and so 
forth ; but which may also be due to any of the number- 
less and hidden causes that are concerned with variation 
in general.? 
I trust, then, that reasons enough have now been given 
to justify my view that, if we take a broad survey of all 
the facts bearing on the question, it becomes almost im- 
possible to doubt that the primary specific distinction is, 
as a general rule, the primordial distinction. I say “as 
a general rule,” because the next point which I wish to 
present is that it constitutes no part of my argument to 
deny that in some—and possibly in many—cases the 
primary distinction may have been superinduced by the 
secondary distinctions. Indeed, looking to the occasional 
appearance of partial sterility between our domesticated 
productions, as well as to the universally high degree of it 
between genera, and its universally absolute degree 
between families, orders, and classes, I see the best of 
reasons to conclude that in some cases the sterility 
between sfecies may have been originally caused, and 77 
a much greater number of cases subsequently intensified, 
by changes going on in other parts of the organism. 
Moreover, I doubt not that of the agencies determining 
such changes natural selection is probably one of the 
most important. But what does this amount to? It 
amounts to nothing more than a re-statement of the theory 
of physiological selection. It merely suggests hypotheti- 
cally the cause, or causes, of that particular variation in 
the reproductive system with which alone the theory of 
physiological selection is concerned, and which, as a 
matter of fact, owsoever caused, is found to constitute the 
one cardinal distinction between species and species. 
Therefore 1 am really not concerned with what I deem 
the impossible task of showing how far, or how often, 
natural selection—or any other cause—may have induced 
this particular kind of variation in the reproductive 
system by its operations on other parts of an organism. 
Even if I were to go the full length that other evolu- 
tionists have gone, and regard this primary specific dis- 
tinction as in all cases due to the secondary specific dis- 
tinctions, still I should not be vacating my theory of 
physiological selection: I should merely be limiting the 
possibilities of variation within the reproductive system 
* Mr. Darwin himself does not appear to have held the view against 
which I am now arguing—v.z. that the primary distinction is always, or 
usually, superinduced by the secondary. Not even here, therefore, is his 
eg opposed to imy views: upon this question his voice is merely 
silent. 
Sse 
NATURE 
aoe 
in what I now consider a wholly unjustifiable manner. 
For, as previously stated, it appears to me much the more 
rational view that the primary specific distinction is like- 
wise, as a rule, the primordial distinction ; and that the 
cases where it has been superinduced by the secondary 
distinctions are comparatively few in number.t 
If we thus regard sterility between species as the result 
of what I have called a local variation, or a variation 
arising only in the reproductive system—whether this be 
induced by changes taking place in other parts of the 
organism, to changes in the conditions of life, or to 
changes inherent in the reproductive system itself—we 
can. understand why such sterility rarely, though some- 
times, occurs in our domesticated productions ; why it so 
generally occurs in some degree between species; and 
why as between species it occurs in all degrees. 
It rarely occurs in our domesticated productions 
because it has never been the object of breeders or horti- 
culturists to preserve this kind of variation, Yet it some- 
times does occur in some degree among our domesticated 
productions, because the changes produced on other 
parts of the organism by artificial selection do, in a small 
percentage of cases, react upon the reproductive system 
in the way of tending to induce sterility with the parent 
form, while not lessening fertility with the varietal form. 
Again, this particular condition of the reproductive 
system is so generally characteristic of species simply 
because in as many cases as it occurs it has constituted 
the reason why species exist as species. And, lastly, this 
particular variation in the reproductive system has taken 
place under nature in such a variety of degrees—from 
absolute sterility between species up to complete, or even 
to more than complete, fertility—because natural species, 
while being records of this particular &ézd of variation 
are likewise the records of all degrees of such variation 
which have proved sufficient to prevent overwhelming 
intercrossing with parent forms. Sometimes this degree 
has been less than at other times, because other con- 
ditions—climatic, geographical, habitatorial, physiologi- 
cal, and even psychological 2—have co-operated to prevent 
intercrossing, with the result of a correspondingly less 
degree of sterility being required to secure a differentia- 
tion of specific type. Lastly, where species have been 
evolved on different geographical areas, or by use, disuse, 
and other causes of a similarly “direct ” kind, there has 
been no need to prevent intercrossing in any degree ; so 
that allied species formed under any of these conditions 
may still remain perfectly fertile, or even more than 
naturally fertile, with one another. 
In view of these considerations, I should regard it as 
a serious objection to my theory if it could be shown that 
sterility between allied species is invariably absolute, or 
even if it could be shown that there are no cases of unim- 
paired fertility. What my theory would expect to find is 
exactly what we do find—namely, an enormous majority 
I The paper here develops another line of argument which it is difficult 
to render in abstract. Its object, however, is to show that, even in the cases 
where the primary distinction is superinduced by the secondary—whether 
these cases are, as I believe, ‘‘comparatively few” or comparatively nume- 
rous—my theory is available to-explain why the primary distinction is so 
habitual an accompaniment of the secondary distinctions, of whatever kinds 
or degrees the latter may happen to be. Tor, according to my theory, the 
reason of this association in such cases is that it can only be those kinds and 
degrees of secondary distinction which are able so to react on the repro- 
ductive system as to induce the primary distinction that are, for this 
reason, preserved, Or, otherwise expressed, in cases where the secondary 
distinctions induce the primary, the former owe their existence to the fact 
that they happened to be of a kind capable of producing this particular 
effect. Under this view, even in these cases it is the principles of physio- 
logical selection that have determined the kinds of secondary distinction 
which are allowed to survive. For these principles have, in all such cases, 
selected the particular kinds of secondary distinction which have proyed 
themselves capable of so reacting on the reproductive system as to bring 
about the primary distinction—a general view of the subject which appears 
to be justified by the very general association between the two. 
2 See ‘Origin of Species,” p. 8x, where it is shown that among verte- 
brated animals different varieties of the same species, even when living on 
the same area, frequently exhibit a marked repugnance to pairing with one 
another. In the same passage, it is remarked the different varieties some- 
times occupy different stations. 
