INTRODUCTION. xi 



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 



THE INTERBREEDING OF BIRDS *. 



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 obvious 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 theoiy 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 Hue 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 beJween them can only be subspecih'c. Subspecies may be denned 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. 



