4 THE GROWTH OF GROUPS 



meaning. The word may call up a different picture in 

 the minds of independent persons. De Vries and other 

 writers have emphasized the fact that most of the 

 Linnaean species are divisible, that they are groups of 

 lesser groups. No zoologist will deny that this is also 

 true of the animal kingdom. The tendency at the 

 present day is to split up those species denned by 

 the older naturalists into lesser groups and to regard the 

 latter as species. But there is much difference of opinion 

 in regard to the reality of a species. There are two 

 opinions, namely, that they are real, and that they are 

 conventional. De Vries was unable to distinguish be- 

 tween the members of certain groups of plants and 

 considered those groups as elementary or indivisible. He 

 therefore regards species as real. An opinion on this 

 question can only be formed by examining and comparing 

 the attributes of mature organisms, and the opinion will 

 vary according to the part of the living kingdom which is 

 examined. An observer, while regarding the higher plants 

 or simpler kinds of animals, may be sure that indivisible 

 groups are appreciable ; but if he turns his attention to 

 vertebrate animals he will doubt whether his former 

 conclusion is applicable to them. When dealing with such 

 complex forms, he may feel inclined to say, " Give me a 

 thousand individuals of any of your so-called elementary 

 species and I will split them into lesser groups by taking 

 into account certain minute characteristics." The struc- 

 ture and function of a vertebrate animal are so complex that 

 it appears as an illimitable field for observation, in which 

 one can explore and describe up to a point, depending 

 upon the acuteness of the observer and the means of 

 discovery. When dealing with a collection of mammalian 



