SPECIES AND VARIETY. 22/ 



and directed in sexual intercourse by the will of their mas- 

 ter, the dogs vary in colour ; in the quantity of hair, which 

 is sometimes entirely lost ; in their nature and properties ; 

 in size, which may differ as one to five in linear dimensions, 

 or more than one to a hundred in the mass ; in the form of 

 the ears, nose, tail ; in the height of the limbs ; in the de- 

 velopement of the brain, and consequent form of the 

 head, which may be slender, with elongated muzzle and flat 

 forehead — or short, with convex forehead ; so that the ap- 

 parent differences between a mastiff and a spaniel, a grey- 

 hound and a poodle, are greater than we find between any 

 wild species of the same natural genus. Lastly, which is 

 the maximum of variation hitherto known in the animal 

 kingdom, there are races of dogs with an additional toe 

 and corresponding metatarsal bone on the hind foot, as there 

 are six-fingered families in the human species. Still, in all 

 these variations, the relations of the bones remain the same, 

 and the form of the teeth is never altered *." 



Thus we find that species must be taken in very different 

 acceptations in wild and domestic animals ; — that while all 

 the beings included under the same species exhibit, in the 

 former case, a close and rigorous resemblance, admitting 

 at most of slight diversities in colour, fur, size, and deve- 

 lopement of some less important parts 5 wider deviations 

 are allowed in the latter, than are observed between some 

 wild animals acknowledged to belong to different species. 



It may be stated, in the abstract, that all animals which 

 differ in such points only as might arise in the natural 

 course of degeneration, that is, from recognized causes of 

 variation, belong to the same species; while those diffe- 

 rences which cannot be accounted for on this supposition 

 must lead us to class the animals which exhibit them in 

 different species. But the chief difficulty is to point out 

 the characters by which, in actual practice, mere varieties 

 may be distinguished from genuine specific differences. 



The transmission of specific forms by generation, and the 

 aversion to unions with t hose of other kinds, soon led natu- 



* CuviEU Rtschcrchcs siir les Osstmens Fossiles ; i. Disc. Prelim, p. 7S. 

 Q 2 



