60 ANIMAL INDIVIDUALITY [OH. 



expose what to the best of our belief are the main 

 steps it has taken. 



Every organism has a general scheme of archi- 

 tecture which can be seen behind the mass of minor 

 adaptive details. It is easy for instance to recog- 

 nize the vertebrate plan in such different-looking 

 creatures as a giraffe, a sparrow, and a sunfish, or 

 the insect plan in butterflies and fleas. With such 

 a plan to start from, change may work in three main 

 ways. First, it may run through the variations on 

 the original plan, without introducing any new com- 

 plication. The different species of a genus, for 

 instance, usually differ from each other in this way. 

 Every one can recognize that polar bear and brown 

 bear and grizzly bear are all built on the same bear- 

 plan, though no one can say that one is better, more 

 differentiated, than another. In the second way the 

 original type of plan is retained, but complications 

 are introduced which imply true differentiation of 

 parts and division of labour ; such parts have never 

 been free and independent, so that the division of 

 labour is very different, in origin especially, from 

 that of insect communities or our human society, 

 where the parts themselves begin as independent 

 individuals. This is not mere change for change's 

 sake, but change progressive. We may call this 

 method internal differentiation, implying that all 

 has taken place within the original unit. An 



