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 subspecific. 
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, &. 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, Xe. 
English ornithologists have for the most part ignored these intermediate 
forms, and with characteristic insular arrogance have sneered at their 
American confréres for adopting trmomial 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 publ- 
cations on ornithology which are not behind the age in this respect. 
ena ts NE Ca Nay te 
