August 12, 1886] 
NATURE 33 
7 
Pore] 
Again, as to plants and animals in a state of nature, 
the particular variation with which we are concerned 
would scarcely be noticed until it had given rise to a new 
species. In this respect, therefore, the theory of physio- 
logical selection is in the same predicament as that of 
natural selection: in neither case are we able directly to 
observe the formation of one species out of another by 
the agency supposed ; and, therefore, in both cases our 
belief in the agency supposed must to a large extent de- 
pend on the probability established by general considera- 
tions. Nevertheless, although our sources of direct 
evidence are thus seen to be necessarily limited, I shall 
now hope to show that they are sufficient to prove the 
only fact which they are required to prove—namely, that 
the particular kind of variation, on the occurrence of 
which my theory depends, does occur both in nature and 
under domestication. 
One very obvious example of the particular variation 
which is required by the theory of physiological selection 
has already been given in the season of flowering or of 
pairing being either advanced or retarded. This I take 
to be a most important case for us, inasmuch as it is one 
that must frequently arise in nature. Depending as it 
chiefly does on external causes, numberless species both 
of plants and animals must, I believe, have been segre- 
gated by its influence. For in every case where a change 
of food, temperature, humidity, altitude, or of any of the 
other many and complex conditions which go to consti- 
tute environment—whether the change be due to migra- 
tion of the species or to alterations going on in an area 
occupied by a stationary species—in every case where 
such a change either retards or promotes the season of 
propagation, there we have the kind of variation which is 
required for physiological selection. And it is needless to 
give detailed instances of such variation where this is due 
to so well known and so frequently observed a cause. 
But it is on what may be called the spontaneous varia- 
bility of the reproductive system itself that I mainly rely 
for evidence of physiological selection. The causes of 
variability are here much more numerous, subtle, and 
complex than are such extrinsic causes as those just 
mentioned ; and they are always at work in the repro- 
ductive systems of all organisms. The consequence is, 
as Mr. Darwin has shown by abundant evidence, that 
variations in the direction of sterility depend more on 
what he calls the nature of the organism than on the in- 
fluence of external conditions. Of this fact we have 
direct evidence, firstly in individuals, secondly in varieties, 
and thirdly in species. 
(1) Zndzviduals.—Mr. Darwin observes, “it is by no 
means rare to find certain males and females which will 
not breed together, though both are known to be perfectly 
fertile with other males and females. We have no reason 
to suppose that this is caused by these animals having 
heen subjected to any change in their habits of life ; there- 
fore such cases are hardly related to our present subject. 
The cause apparently lies in an innate sexual incompati- 
bility of the pair when matched.” He then proceeds to 
give examples from horses, cattle, pigs, dogs, and pigeons, 
concluding with the remark that “these facts are worth 
recording, as they show, like so many previous facts, on 
what slight constitutional differences the fertility of an 
animal often depends.” Elsewhere he gives references 
to similar facts in the case of plants ; and instances of 
this individual incompatibility, both in plants and animals, 
might easily be multiplied. 
Now, if even as between two individuals there may thus 
arise absolute sterility without there being in either of 
them the least impairment of fertility towards other in- 
dividuals, much more may such incompatibility extend 
towards a number of individuals. For certainly the most 
remarkable feature about this individual incompatibility 
is the fact of its being only individual: it would not be 
nearly so remarkable, or antecedently improbable, if the 
incompatibility were to run through a whole race or strain. 
In the fact of individual incompatibility, therefore, we 
have the kind of variation which my theory requires, and 
this as arising spontaneously in the highest degree of 
efficiency. 
(2) Races.—But of even more importance for us is the 
direct evidence of such a state of matters in the case of 
varieties, breeds, or strains. In the ninth chapter of the 
“ Origin of Species,” and in the nineteenth chapter of the 
“‘Variation of Plants and Animals under Domestication,” 
Mr. Darwin adduces miscellaneous instances of varieties 
of the same species which exhibit a higher degree of fer- 
tility within themselves than they do with one another. 
In this respect, therefore, they resemble natural species ; 
but as they are only domesticated varieties known to 
belong to the same species, they are here available as 
evidence of what may be termed racial incompatibility, or, 
of the particular kind of variation which my theory re- 
quires. ‘To quote only two instances: “The yellow and 
white varieties (of Verbascum) when crossed produce less 
seed than the similarly coloured varieties”; and the blue 
and red varieties of the pimpernel are absolutely sterile 
together, while each is perfectly fertile within itself. Such 
instances are the more suggestive on account of their 
arising under domestication, because, as a rule, domesti- 
cation increases fertility, and is thus inimical to sterility— 
sometimes even breaking down the physiological barriers 
between natural species. Therefore, if in some cases even 
under domestication the reproductive system may vary in 
this manner, so as to erect physiological barriers between 
artificial varieties, much more are such barriers likely to 
be erected between varieties when these arise in a state 
of nature. 
But as regards varieties in a state of nature, I have not 
been able to meet with any evidence of racial incom- 
patibility. Nor is this to be wondered at : for, unless the 
degree of such incompatibility were well pronounced, it 
would not be noticed ; while, if it were well pronounced, 
the two varieties would for this very reason be classified 
as species. Therefore, the fact of racial incompatibility 
within the limits of wild species could only be proved by 
experiments undertaken expressly for the purpose, in the 
way which I shall afterwards explain. 
(3) Sfectes—According to the general theory of evolu- 
tion, which in this paper is taken for granted, the distinc- 
tion between varieties and species is only a distinction of 
degree ; and the distinction is mainly, as well as most 
generally, that of mutual sterility. Therefore my theory of 
physiological selection is here furnished with an incal- 
culable number of instances of the particular kind of 
variation which is required ; for in so many instances as 
variation has led to any degree of sterility between parent 
and varietal forms—or between the varying descendants 
of the same form—in so many instances it is merely a 
statement of fact to say that physiological selection must 
have taken place. There remains, however, the question 
whether the particular change in the reproductive system 
which led to all these cases of mutual sterility was an- 
terior or posterior to changes in other parts of the 
organism. For, if it was anterior, these other changes— 
even though they be adaptive changes—were presumably 
due to the sexual change having interposed its barrier to 
crossing with parent forms; while, if the sexual change 
were posterior to the others, the presumption would be 
that it was the latter which, by their reaction on the sexual 
system, induced the former. I shall have to consider 
this alternative later on. Here, therefore, it is enough 
to point out that under either possibility the principles of 
physiological selection must have been at work ; only these 
principles are accredited with so much the more causal 
influence in the production of species in the proportion 
that we find reason to suppose the sexual change to have 
been, as a rule, the prior change. But under either al- 
ternative, and on the doctrine that species are extreme 
