140 ANIMAL INDIVIDUALITY [CH. 



comes the internal differentiation of this individual : 

 blood-cells and blood-vessels, nerve-cells and sense- 

 organs, muscles, sinews, bone, kidney, liver kind 

 after kind of cell is created. And let it never be 

 forgotten that in the embryonic development of any 

 and every individual all these and many others are 

 descendants of a single and a simple cell. 



It is noteworthy that the course of internal 

 differentiation has over and over again in worms, 

 in insects, in Crustacea, in spiders, in molluscs, and in 

 vertebrates tended in the same direction towards 

 the formation of a Brain. Brain-development has 

 usually gone hand in hand with the specialization of 

 other organ-systems the brain seems a mere bit 

 of machinery necessitated b} r the complexity of the 

 other parts. In the higher insects and the higher 

 mammals, however, the brain seems to have tran- 

 scended all the other parts of the body, to have gone 

 farther than they in specialization, and to be now 

 in truth the master by whom the rest are to be 

 employed. 



This development of sense-organs and brain has 

 had great influence on the progress of individuality. 

 We do not usually stop to consider in what dense 

 darkness the majority of living things must live and 

 move and have their being. Without brain or sense- 

 organs, theirs must be a dim and windowless existence. 

 The world lies waiting round about ; but it cannot 



