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INTRODUCTION. x1 
two species are only half evolved. Do we find these half-formed species ? 
At any period. of the world’s history, if the process of evolution is 
always going on, there ought surely to be some instances of half-evolved 
species. So there are. It is easy to find examples of species in every 
stage of development, from mere local races to well-defined subspecies. 
To enable us to discriminate between these on the one hand and between 
species and subspecies on the other, it is necessary to inquire into 
Tuer INTERBREEDING oF Birps*. 
This is a subject which has been much neglected by ornithologists. 
The existence of intermediate forms so produced has been as much as 
possible ignored. Where the facts were too obyious to admit of doubt, 
the so-called cross was contemptuously dismissed as a hybrid, a mon- 
strosity, and as such possessing no more scientific interest than a white 
Blackbird or a six-legged calf. So long as each species was supposed to 
have had a separate origin, and to be divided by a hard-and-fast line from 
every other species, this attitude of ornithologists towards interbreeding 
was excusable; but now that the theory of evolution has been generally 
accepted, the subject will be found to possess the greatest interest and to 
throw unexpected light upon the development of species. The old defini- 
tion of species having lapsed, in consequence of the rejection of the theory 
of special creation, it is necessary to provide a new one. We may define 
a species to be a group of individuals which, however much they may vary 
from each other, do not present any hard-and-fast line between their 
extreme variations, and which, however near they may be to their nearest- 
allied species, are nevertheless separated from them by a hard-and-fast 
line. Naturalists may differ as to the assignment of the cause why inter-— 
mediate forms are absent; but we may reasonably infer, first, that the 
intermediate forms have become extinct, and, secondly, that they are not 
reproduced by interbreeding. There may be several reasons why they are 
not reproduced by interbreeding. Where Nature has drawn the line very 
broadly, the species may have been so long separated and may have become 
so differentiated that productive sexual intercourse between them may 
have become structurally impossible. A somewhat narrower line exists 
between species which may be artificially crossed, but produce under those 
circumstances only a barren hybrid. The specific line of demarcation is 
* Interbreeding may or may not mean cross-breeding. Wherever the interbreeding 
which habitually takes place between the individuals of a species has not ceased, any 
differences between them can only be subspecific. Subspecies may be defined as groups 
in which the interbreeding which habitually takes place between individuals in a species 
has not yet ceased, but takes place along the whole line of its geographical distribution, 
though seldom between the two extremes, 
