64 



ON SUBSPECIFIC FORMS. 



Practical 

 difference 

 between 

 species and 

 subspecies. 



Clearheaded- 

 ness of 

 American 

 ornitholo- 

 gists on this 

 point. 



breeding is reached in species which, when artificially crossed in confinement, produce a 

 more or less fertile hybrid, but which do not do so in a wild state, either because the 

 inclination to cross is absent, or because the opportunity of doing so is prevented by the 

 isolation of their respective geographical areas. The first stage of interbreeding, as 

 distinguished from intercrossing, is represented by two subspecifically distinct forms, 

 having isolated areas of distribution, but where the difference between them is so slight 

 that the ranges of their respective variations overlap ; so that in the case, for example, 

 of a large continental form and a small island form, the smallest continental examples 

 are indistinguishable from the largest insular examples, and which would presumably 

 interbreed if they had an opportunity of doing so. The final stages of sub-specific forms 

 are represented by species so closely allied that they habitually interbreed whenever the 

 areas of their distribution overlap, and habitually produce offspring of various degrees of 

 fertility — a condition of things which, if the fertility continue for a sufficient number of 

 generations, must inevitably produce an unbroken series of intermediate forms. 



Where the area of distribution is small, the interbreeding which consequently takes 

 place must soon produce a homogeneous but intermediate species. It is probable, however, 

 that the formation of a new species from the blending of two forms which have 

 emigrated to the same locality has not been of very frequent occurrence. Intermediate 

 forms generally occur in the middle of the area of distribution of a species which has a 

 very wide range, and which has been modified in different directions at each extremity of 

 its range, sometimes to such an extent that the individuals at the two extremes have become 

 specifically distinct from each other, though remaining only subspecifically distinct from the 

 intermediate forms which connect them. 



In practice it will be found that the most convenient line that can be drawn between 

 a species and a subspecies is to regard two forms as specifically distinct, however near they 

 may be to each other, whenever they are not connected by intermediate forms ; and to 

 regard two forms as only subspecifically distinct, however wide may be the distance between 

 the extremes, whenever they are connected by a series of intermediate forms — without 

 reference in the one case to how the intermediate forms are produced, or in the other to 

 why they are not produced. 



It is only doing scant justice to American ornithologists to admit that to them 

 belongs the credit of having for the first time formed a clear conception of the difference 

 between a species and a subspecies, and of having at once recognized the fact in a scientific 

 manner in their nomenclature. It may be a moot point whether the use of trinomials or 

 polynomials be the best mode of expressing the recognition of subspecies. That is a 

 matter of detail which is only of secondary importance. The primary truth, the recognition 

 of which in some way or other is of vital importance to a clear understanding of the facts 

 of Zoology, is that species in the process of differentiation do exist in considerable 

 numbers. 



The American ornithologists were not the first writers on birds who used a trinomial 



