144 THE SHAPES AND SIZES OF ANIMALS 



forms are undoubtedly the Cephalopoda, and it is 

 amongst these that the largest members of the 

 group occur. 



It has been suggested recently that the rule is a 

 stricter one than has been hitherto recognised ; 

 and evidence has been quoted in support of the 

 further statement that the ancestral forms or 

 progenitors of the higher groups, such as mammals 

 or birds, were of distinctly small size, i.e., much 

 below the average stature attained by their 

 descendants the present members of the groups 

 in question. The direct influence of size on 

 structure has as yet been but very imperfectly 

 investigated, but there are many cases known in 

 which among animals of the same zoological group 

 the larger forms are distinctly of more complicated 

 structure than their smaller allies, and in which 

 there are very valid grounds for holding that the 

 greater complication of structure is connected 

 casually with the increased dimensions. 



We shall perhaps best deal with the several 

 points just mentioned by taking the large groups 

 into which the animal kingdom is divided one by 

 one, and noticing with regard to each, the principal 

 facts concerning the sizes of the several members of 

 the group. 



Protozoa, the simplest of all animals, are defined 

 as those forms which, not merely in the earliest 

 phases of their existence, but throughout their 

 entire lives, remain in the condition of single 

 cells. This unicellular nature is associated with 

 great simplicity of organisation and with extremely 



