I.] INTRODUCTION. 



XIX 



nate in some peculiarity of climate, situation, composition 

 of the soil or water which they inhabit, the nature or 

 supply of food, and various other conditions. The cha- 

 racters by which they usually differ from species consist 

 of size, comparative proportions of different parts, colour, 

 and degree of sculpture ; and the investigation of forms 

 thus changed or modified is often extremely perplexing. 

 Some species are more liable than others to variation ; 

 and, as might be expected, the tendency to variation is 

 greater in those species which most abound in individuals, 

 by reason of their offering a wider scope of observation 

 to the naturalist. Varieties are of two kinds, perma- 

 nent and local. The former are called "races," and 

 have many of the characters of true or typical species, 

 with which they associate. Great experience and good 

 faith are essential to the investigation of this experi- 

 mentum crucis ; and the only reliable test of distinction 

 between species and races appears to be the discovery 

 of intermediate forms. In the absence of such discovery 

 we may proceed to classify, although after all we must 

 not lose sight of the great probability that all species, 

 and even higher groups, may have originally descended 

 from races or permanent varieties, and these again from 

 local varieties. The latter are more readily distinguished 

 from species and are never found associated with them. 

 I believe it may be now considered a well-established 

 rule, that all distinct groups of individuals living toge- 

 ther and having a common feeding-ground, and which 

 are not connected or blended with each other by insen- 

 sible gradations, are primd facie entitled to the rank of 

 species. A contrary opinion used formerly to be enter- 

 tained by some naturalists ; and it was not unusual 

 to found a claim to specific distinction on the fact that 

 the specimens thus distinguished did not occur with the 



