116 STUDIES IN ANIMAL LIFE. 



ence. Only individuals exist, and these all vary 

 more or less from each, other. When the modifica- 

 tions are slight, they have no name ; when they are 

 more marked, and are transmitted from one gener- 

 ation to another, they constitute particular races or 

 varieties ; when the differences are still more mark- 

 ed, they constitute sub-species ; but, as Mr. Darwin 

 says, " Certainly no clear line of demarkation has 

 yet been drawn between species and sub-species; 

 that is, the forms which in the opinion of some nat- 

 uralists come very near to, but do not quite arrive 

 at the rank of species ; or again, between sub-spe- 

 cies and well-marked varieties, or between lesser 

 varieties and individual differences. These differ- 

 ences blend into each other in an insensible series ; 

 and a series impresses the mind with the idea of an 

 actual passage." But the same process of diverg- 

 ence which establishes varieties out of individual 

 differences, and species out of varieties, also serves 

 to establish genera out of species, orders out of gen- 

 era, and classes out of orders. It is doubtless diffi- 

 cult to conceive by what process of modification 

 two animals of distinct genera, say a dog and a cat, 

 were produced from a common stock ; but organic 

 analogies in abundance render it easy of belief. If 

 we knew as much of zoology as we do of embryol- 

 ogy, in respect of the affinities of divergent forms, 

 it would be far less surprising that two different 

 genera should arise from a common stock, than that 

 all the various parts of the skeleton should arise 



