The term "species" was at first used very loosely by scientific writers.
It meant very little more than our vague word _kind_ does at the
present time. Not until the time of Linnæus (1707-1778) did the term
acquire a definite and precise meaning. The aphorism of the great
botanist, "_species tot sunt diversæ quot diversæ formæ ab initio sunt
creatæ_"--"just so many species are to be reckoned as there were forms
created in the beginning,"--was at least an attempt to use the term in a
well-defined sense. Of course, this definition assumed the "fixity" of
species; but with the wide prevalence of the views of Darwin and his
followers the term "species" has fallen into disrepute, and is now
regarded by many as only an artificial rank in classification
corresponding to no objective reality in the natural world. Some
writers, as Lankester, have found so much fault with the term as to urge
its complete abandonment in scientific literature. This is logical
enough from the standpoint of Darwinism; for if the latter be true there
ought indeed to be such a swamping of every incipient "species" as to
make one kind blend with others all around it in the classification
series.
But since the term has by no means been discarded, we must endeavor to
determine the sense in which it continues to be used in good scientific
literature.
"A species," says Huxley, "is the smallest group to which distinct and
invariable characters can be assigned." The Standard Dictionary says
that the term is used for "a classificatory group of animals or plants
subordinate to a genus, and having members that differ among themselves
only in minor details of proportion and color, and are capable of
fertile interbreeding indefinitely."
The latter authority also adds:
"In the kingdoms of organic nature species is founded on identity of
form and structure, and specifically characterized by the power of the
individuals to produce beings like themselves, who are in turn
productive."
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.]