Interbreeding of Birds. 547 



The old definition of a species having lapsed, in consequence 

 of the rejection of the theory of special creation, it is neces- 

 sary to provide a new one. The first step towards an under- 

 standing of what constitutes a species is the admission of the 

 existence of subspecies. Two forms which are apparently 

 very distinct, as Corvus corone and C. comix or Carduelis 

 major and C. caniceps, are nevertheless found to be only sub- 

 specifically distinct — a complete series of examples from one 

 extreme form to the other in each case being obtainable. 

 These are produced by interbreeding. In the case of the 

 Crows it has been proved over and over again that the two 

 extreme forms not only interbreed with each other, but also 

 with the intermediate forms ; so that not only are mulattos 

 produced, but also quadroons, octoroons, &c. Of course, in 

 no other way could a complete series from one extreme form 

 to the other be obtained. 



In some genera of birds we find the relationship between 

 the species still more complicated, hanius excubitor inhabits 

 Western Europe : it is an intermediate form between L. major 

 o£ North-eastern Europe and Siberia and L. leucopterus of 

 South-eastern Europe and Siberia. A complete series of 

 examples of intermediate forms connecting L. major and 

 L. leucopterus may be obtained ; and yet both species inhabit 

 the same district in Siberia and appear to be specifically 

 distinct, no intermediate forms having been obtained from 

 that country. On the other hand, both the extreme forms 

 appear to be only subspecifically distinct from L. excubitor, 

 inasmuch as in North-eastern Europe every intermediate 

 form is found between L. major and L. excubitor, and in 

 South-eastern Europe every intermediate form is found 

 between L. excubitor and L. leucopterus. In this case we 

 may assume that L. excubitor was the original Shrike from 

 which L. major and L. leucopterus have varied in opposite 

 directions. In the case of the Crows and the Goldfinches, 

 already cited, the circumstances are probably somewhat 

 diflPerent. The original Crow or Goldfinch may have been 

 an intermediate form of either of the extreme forms; but 

 there is considerable evidence to prove that, in the case of the 



