10 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, 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 
ofthe 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, and alti- 
