xii INTRODUCTION. 



still narrower where barren hybrids are produced in a state of Nature. 

 The line of demarcation is considerably narrowed when more or less fertile 

 hybrids can be artificially produced but do not occur in a wild state, either 

 because the natural inclination to interbreed is absent or because the 

 opportunity of interbreeding is taken away by isolation of area of geogra- 

 phical distribution ; and we may consider the narrow line between such 

 species and subspecies to be crossed when fertile hybrids are produced in 

 a state of nature a condition of things which, if the fertility is sufficient 

 to continue to many generations, must inevitably produce an unbroken 

 series of intermediate forms. " The amount of sterility," says Darwin, 

 " between any two forms when first crossed, or in their offspring," which 

 shall be " considered as a decisive test of their specific distinctness " is a 

 point upon which naturalists are not agreed. There is no hard-and-fast 

 line between a specific difference and a difference which is only snbspecific. 

 The practical result is that slight subspecific variations are constantly being 

 produced by various causes, of which natural selection is probably the 

 most important, and are as constantly being lost by interbreeding ; so that 

 the similarity of individuals in a species is retained, whilst the sterility 

 produced by a specific variation prevents the universal mongrelization of 

 species which might otherwise take place. Interbreeding is a check upon 

 the indefinite multiplication of species ; whilst the narrow limit in which 

 it is possible provides against the extinction of specific differences. 

 Amongst British birds there are a great many instances of subspecies 

 of which we know, and no doubt many more of which we do not yet 

 know. Most of these are cases where the individuals of each valley 

 occasionally interbreed with their immediate neighbours, and where the 

 range is great enough to make the sum of a series of small differences 

 show a large difference in the extremes, as the Nuthatch, Marsh-Tit, 

 Grey Shrike, &c. Others are cases where the species appear to be per- 

 fectly distinct, but nevertheless it is found that, where their respective 

 ranges meet, they interbreed and produce offspring which are fertile both 

 among themselves and with either parent, as the Dipper, Goldfinch, 

 Crow, &c. 



English ornithologists have for the most part ignored these intermediate 

 forms, and with characteristic insular arrogance have sneered at their 

 American confreres for adopting trinomial names which their recognition 

 demands. In this, as in so many other things, our American cousins are 

 far in advance of the Old World. One English ornithologist, however, 

 deserves to be mentioned as an honourable exception. Mr. Bowdler 

 Sharpe has boldly braved the blame of the Drs. Dry-as-dust and the 

 Professors Red-tape ; and the volumes of the ' Catalogue of Birds in the 

 British Museum' hitherto represent almost the only European publi- 

 cations on ornithology which are not behind the age in this respect. 



