Ixviii INTRODUCTION. 



ral type, and there is, thus far, no distinction between the va- 

 riety and the species. It may be said, indeed, that the charac- 

 ters of the species are more lasting than those of the varie- 

 ty : but, unless we are to assume that the forms of animals are 

 immutable, this is a difference in degree and not in kind ; 

 and a variety, therefore, does not differ in kind from a spe- 

 cies. It may readily be supposed, then, that with respect to 

 certain animals, questions may arise, whether they be species 

 or varieties. But if the only real difference between a spe- 

 cies and variety be, that the characters of the one are more 

 lasting than those of the other, innumerable cases must pre- 

 sent themselves, in which we cannot determine whether a 

 given animal be what we call a species or a variety. Yet 

 eager debates are continually carried on by naturalists whe- 

 ther certain animals are to be regarded as species, or as 

 varieties. Thus, the Common Wolf of America differs some- 

 what in aspect from the Wolf of Europe, and some natu- 

 ralists hold that he is specifically distinct ; but all that we 

 can truly say is, that the wolf of Europe and the wolf of 

 America present varieties of that form which we term Wolf, 

 and our knowledge of the animal conducts us no further. 

 The Domesticated Dogs present greater varieties of form 

 and characters than many animals which are considered 

 to be specifically different. The question has arisen whe- 

 ther these dogs are of different species or of one species? 

 The resolution of the question, it is manifest, depends mainly 

 upon the meaning which we assign to our own terms. If we 

 are to include, under the same specific form, the long muzzle 

 and slender limbs of the Greyhound, and the short muzzle 

 and stout limbs of the Bull-dog, then the Greyhound and the 

 Bull-dog are of one species ; if we hold that the elongated 

 muzzle and slender limbs of the one constitute a specific dis- 

 tinction, then the Greyhound and the Bull-dog are of different 

 species according to our definition. 



But a species, it has been supposed, difl'ers from a variety 

 in this, that while animals of different species will not breed 



