34 THE THEORY OF EVOLUTION 



lists, because of the manner in which the different 

 individuals of a species vary among themselves and, 

 in many instances, of the way in which the species 

 of a genus grade into each other. No two individuals, 

 belonging to the same species, are exactly alike in all 

 respects and when a very large number of such in- 

 dividuals are compared, they are found, as a rule, 

 to cover a remarkably wide range of variation, the 

 extremes of which often differ more than do the in- 

 dividuals which are referred to distinct species. The 

 species of a large, thriving and vigorous genus are 

 often so unstable and so connected by intergrada- 

 tions, that there is sometimes a radical difference of 

 opinion as to how many separate species should be 

 recognized. "Nothing is brought out more clearly 

 by ecological studies in New Zealand than the ex- 

 treme * plasticity' of many species and structures, 

 and their rapid response to a change of environment. 

 This is so great in numerous instances that the idea of 

 'normal' loses its meaning." l 



For example, the botanists differ greatly in regard 

 to the American oaks and into how many species 

 they are properly divisible; the same is true of many 

 other groups. To evade this difficulty, at least in 

 part, several groups, subordinate to species, are recog- 

 nized and named in systematic botany and zoology, 

 such as geographical races, varieties, subspecies, etc., 

 each of these minor groups being relatively constant 

 and recognizable, yet not differing sufficiently from 



1 L. Cockayne in Trans. New Zealand Institute. Vol. XLIV, pp. 13, 14. 



