244 NATURAL HISTORY. 



stumbling-block of systematic zoologists. The general 

 ideas which are in the minds of naturalists when they 

 speak of ' species ' are expressed by the definition, that 

 a ' species' of animals consists of an assemblage of indi- 

 viduals^ all resembling each other, and producing their like by 

 generation. Thus, all wolves* resemble each other and 

 produce fertile young; hence wolves constitute a single 

 'species,' the Cams lupus of Linnaeus. Similarly, the 

 individuals of the lion, tiger, brown bear, and so on, 

 resemble each other and produce fertile young, thus con- 

 stituting so many distinct ' species ' of animals. There is, 

 however, the obvious difficulty that the individuals of any 

 species do not altogether resemble each other. They 

 resemble each other in generals, but not in particulars. 

 All wolves are alike, but they are not absolutely alike. 

 Besides, in many cases the differences which exist in the 

 individuals which compose any given species may be 

 very considerable; and so far as our observation goes, 

 they may be permanent differences. This is most con- 

 spicuously the case among our domesticated animals, such 

 as the dog or pig. Such species are known by every 

 one to contain certain groups of individuals which differ 

 extraordinarily from each other ; and the differences 

 distinctive of these groups are to all appearance as per- 

 sistent as the differences which separate distinct species. 

 Thus, to take the case of dogs, the mastiff, greyhound, 

 bulldog, terrier, and so on represent such groups of differ- 

 ing individuals, which in common language are known 

 as different ' breeds ' or ' races ' of dogs. Such breeds are 

 in some cases known to have existed without notable 



* The European wolves, of course, are meant. 



