Species

Darwin and the Darwinians 173

And, reiterating the comment in the Notebook:

No one has been able to point out what kind or what amount of difference, in any rec-

ognisable character, is sufficient to prevent two species crossing.^66

Darwin notes that species are almost always intersterile, but that this is often due
to the fact that as soon as this intersterility is noticed, taxonomists will rank these
varieties as species:

It may be urged, as an overwhelming argument, that there must be some essential

distinction between species and varieties, inasmuch as the latter, however much they

may differ from each other in external appearance, cross with perfect facility, and

yield perfectly fertile offspring. With some exceptions, presently to be given, I fully

admit that this is the rule. But the subject is surrounded by difficulties, for, looking to

varieties produced under nature, if two forms hitherto reputed to be varieties be found

in any degree sterile together, they are at once ranked by most naturalists as species.^67

Anyway, some obvious varieties within species are sterile together, even though
they are able to breed with mutual races:

From these facts it can no longer be maintained that varieties when crossed are invari-

ably quite fertile. From the great difficulty of ascertaining the infertility of varieties in

a state of nature, for a supposed variety, if proved to be infertile in any degree, would

almost universally be ranked as a species;—from man attending only to external char-

acters in his domestic varieties, and from such varieties not having been exposed for

very long periods to uniform conditions of life;—from these several considerations

we may conclude that fertility does not constitute a fundamental distinction between

varieties and species when crossed. The general sterility of crossed species may safely

be looked at, not as a special acquirement or endowment, but as incidental on changes

of an unknown nature in their sexual elements.^68

So in the end, Darwin refuses to make sterility a test of species, or even to expect
that sterility will correlate with systematic affinity, summarizing the arguments in
that chapter thus:

First crosses between forms, sufficiently distinct to be ranked as species, and their

hybrids, are very generally, but not universally, sterile. The sterility is of all degrees,

and is often so slight that the most careful experimentalists have arrived at diametri-

cally opposite conclusions in ranking forms by this test. The sterility is innately vari-

able in individuals of the same species, and is eminently susceptible to the action of

favourable and unfavourable conditions. The degree of sterility does not strictly follow

systematic affinity, but is governed by several curious and complex laws. It is generally

different, and sometimes widely different in reciprocal crosses between the same two

species. It is not always equal in degree in a first cross and in the hybrids produced

from this cross.

(^66) Op. cit., 215f.
(^67) Op. cit., 226.
(^68) Op. cit., 229.