io INTRODUCTION 



and families) of animals and plants exist, in which 

 the most nearly related species are quite definitely 

 distinct from one another. In other classes systema- 

 tists have so far found great difficulty in framing 

 definitions of specific groups. We shall see later on, 

 though at first sight it may appear almost paradoxical, 

 that it is quite possible for groups to be perfectly 

 distinct j although individual members of them may 

 have deviated so far, each from its proper type, as to 

 render impossible the task of deciding from their 

 appearance which group any of these individuals 

 belong to. 



Let us next consider a particular example of a class 

 of animals in which the discrimination of species is 

 difficult or impossible. This is said to be the fact 

 with the majority of sessile animals such animals 

 as resemble plants in their stationary habit, and in 

 no case are the problems of species separation more 

 difficult than in the class of the stony corals. Now, 

 attempts to determine the species of corals have so 

 far been made almost entirely from a study of what 

 may be called vegetative characters usually from 

 details of the shape and structure of the stony skeleton 

 of the animals. How far these features may be affected 

 by external circumstances has not been determined, 

 but it must be noted that the so-called skeleton is 

 entirely external to the living organism. Now we 

 know that in the case of many of the higher plants 

 vegetative characters are extremely liable to become 

 modified owing to the action of the environment. 

 Differences of moisture $ light, soil, climate j and alti- 



