To put the matter still more definitely before the reader, we quote the
following from a well-known scientist whose writings on the subject of
evolution have had a wide circulation:

"There are two bases on which species may be founded. Species may be
based on _form_, morphological species; or they may be based on
_reproductive functions_, physiological species. By the one method a
certain amount of difference of form, structure, and habit, constitutes
species; according to the other, if the two kinds breed freely with each
other and the offspring is indefinitely fertile, the kinds are called
varieties, but if they do not they are called species."[15]

This author adds that this physiological test, as to whether or not the
kinds are cross fertile, "is regarded as a most important test of true
species, as contrasted with varieties or races."

[Footnote 15: Joseph Le Conte, "Evolution and Religious Thought," p.
233.]


III

When we look at the matter in this light, it is very evident that there
are multitudes of long recognized specific distinctions that ought to
be discarded. For instance, there are some twenty odd "species" of wild
pigs scattered over the Old World, which Flower and Lydekker assure us
would probably "breed freely together."[16] The yak and the zebu of
India, and the bison of America, would on this basis have to be
surrendered, for it is well known that they will all breed freely with