364 
of physiological selection. For, the larger a genus, or 
the greater number of species it contains, the greater 
must be the opportunity afforded for the occurrence of 
that particular kind of variation on which the principle of 
physiological selection depends. All the species of a 
genus may be regarded as so many varieties which have 
already been separated from one another physiologically: 
therefore each of them may now constitute a new starting- 
point for a further and similar separation—particularly as, 
in virtue of their previous segregation, many of them are 
now exposed to different conditions of life. Thus, it 
seems to me, we can well understand why it is that genera 
already rich in species tend to grow still richer ; while 
such is not the case in so great a degree with genera that 
are poor in species. Moreover, we can well understand 
that, multiplication of species being in the first instance 
determined by changes in the reproductive system alone, 
wherever a large number of new species are being turned 
out, the secondary differences between them should be 
“often exceedingly small”—a general correlation which, 
so far as I can see, we are not able to understand on the 
theory of natural selection. 
Another general fact mentioned by Darwin, and now 
well recognised by all naturalists, is that closely allied 
species, or species differing from one another in trivial 
details, usually occupy contiguous areas ; or, conversely 
stated, that contiguity of geographical position is favour- 
able to the appearance of species closely allied to one 
another. Of course this fact speaks in favour of evolu- 
tion ; but where the question is as to method, I confess 
that the theory of natural selection appears to me wholly 
irrelevant. For,in most of the numberless cases to which 
I allude, the points of minute detail wherein the allied 
species differ in respect of secondary distinctions, are 
points which present no utilitarian significance. And, 
as previously argued, it is impossible to believe that 
there can be any general or constant correlation between 
disguised utility and insignificance of secondary dis- 
tinction. 
Now the large body of facts to which I here allude, 
but which I have not space to detail, appears to me to 
constitute perhaps the strongest of all my arguments in 
favour of physiological selection, Take, for instance, a 
large continental area, and follow across it a chain of 
species, each link of which differs from those on either 
side of it by the most minute and trivial distinctions of a 
secondary kind ; but all the links of which differ from one 
another in respect of their reproductive systems, so that 
no one member of the series is perfectly fertile with any 
other member. Can it be supposed that in every case 
this constant primary distinction has been superinduced 
by the trivial secondary distinctions, distributed as they 
are over different parts of all these kindred organisms, 
and yet nowhere presenting any but the most trifling 
amount of morphological change? Or, even if we were 
to suppose this, we have still to meet the question, How 
were all these trifling changes produced in the face of 
free intercrossing on the continental area? Certainly not 
by natural selection, seeing that they are all useless to the 
species presenting them. Let it then be by changes in 
the conditions of life, whether of food, of ciimate, or of 
anything else. I can conceive of no other alternative. 
Yet, if we accept this alternative, we are but espousing— 
in-a disguised and roundabout way, to be sure—the theory 
of physiological selection. For we are thus but hypothetic- 
ally assigning the causes which have induced the primary 
distinction in each case, or the causes which have led to 
the mutual sterility. For my own part, I believe that the 
assignation would be, in the great majority of such cases, 
incorrect. That is to say, 1 do not believe that in the 
great majority of such cases the trivial secondary distinc- 
tions—however these were caused—can have had any- 
thing to do with the great primary distinction. What I 
believe is that all the closely-allied species inhabiting our 
NATURE 
[August 19, 1886 
supposed continent, and differing from one another in so 
many points of minute detail, are but so many records of 
one particular kind of variation having taken place in the 
reproductive systems of their ancestors, and which, so 
often as it did take place, necessarily gave birth to a new 
species. The primary distinction thus became the con- 
stant distinction, simply because it was in virtue of this 
distinction—or in virtue of the variation which first 
originated this distinction—that the species became 
species; and the secondary distinctions thus became 
multitudinous, minute, and unmeaning, simply because 
they were of later origin, the result of spontaneous varia- 
bility, unchecked by intercrossing with the parent forms, 
and, on account of their trivial (¢.e. physiologically harm- 
less) nature, unchecked also by natural selection, economy 
of growth, or any other principle which might have pre- 
vented spontaneous variability of any other kind. 
There are many other general facts relating to geogra- 
phical distribution which lend the strongest countenance 
to the theory of physiological selection—in particular I 
may mention the difficulty which Mr. Darwin experiences 
in accounting for the absence or rarity of transitional 
varieties between species inhabiting contiguous areas (/oc. 
ctt., p. 134), which is just what might have been expected 
on my theory—but it is time that this abstract should 
draw to a close. 
Relations between the Theories of Natural Selection and 
Physiological Selection.—The two theories resemble one 
another in the kind of evidence by which they are each 
supported. For in neither case is this evidence that of 
direct observation of the transmutation of species under 
the influence of the agency supposed: the evidence in 
each case consists in first proving the facts on which the 
principle depends, and then showing that the phenomena 
of organic nature are such as they ought to be if the 
principle in question has had any large share in their 
production. But the two theories differ in that while 
natural selection is a theory of the origin of genera, fami- 
lies, orders, and classes even more than it is of the origin 
of species ; the theory of physiological selection is almost 
exclusively a theory of the origin of species. Again, the 
latter theory differs from the former in that the variations 
on the occurrence of which it depends are variations of 
a comparatively unuseful, or non-adaptive, kind. Never- 
theless, physiological selection must be quite as vigilant 
as natural selection, and it seizes upon the comparatively 
unuseful variation of sterility with even more certainty than 
natural selection canseize uponanyusefulvariation. Lastly, 
as will have been gathered from the foregoing abstract, the 
two theories are in no way opposed to one another: they 
are, in fact, complementary, and the principles with which 
they have to deal co-operative. For, on the one hand, 
without the assistance of physiological selection, natural 
selection would, I believe, be all but overcome by the 
adverse influences of free intercrossing—influences all 
the more potent under the very conditions which are 
required for the multiplication of species by divergence of 
character. On the other hand, without natural selection, 
physiological selection would be powerless to create any 
differences of specific type other than those of mutual 
sterility and trivial details of structure, form, or colour— 
differences wholly without meaning from a_ utilitarian 
point of view. But in their combination these two 
principles appear to me able to accomplish what neither 
can accomplish alone—namely, a full and satisfactory 
explanation of the origin of species. 
Concluston.—It has not been possible to do justice to 
the theory of physiological selection within the limits of this 
abstract. _But perhaps enough has been said to show 
that there is a great deal of evidence in its support ; that 
by regarding mutually sterile species as records of varia- 
tion in reproductive systems, we are at work, so to speak, 
on the foundation of the matter ; and that we are thus 
able to explain a number of general facts which do not 
ee 
