20 ESSAYS OF A BIOLOGIST 



human history in large measure the history of the 

 increase in size of social units? 



But size alone is not enough; there is also a defi- 

 nite improvement of the details of life's mechanism — 

 partly revealed as improvement in the efficiency of 

 the parts themselves, partly in the adjustment of the 

 parts to each other, and their subordination to 

 the needs of the whole. 



It is scarcely necessary to detail the improvements 

 in efficiency of different organs during evolution: 

 such are universally familiar. But a few examples 

 will point my moral. The lowest three-layered 

 forms have no circulatory system; this, rendered nec- 

 essary later by increase of size, shows a gradual 

 differentiation of parts in evolution. The exquisite 

 machinery of our heart is directly descended from a 

 minute pulsating ventral vessel such as that seen in 

 Amphioxus. Protection and support are better 

 cared for in insect than in worm, in mammal than 

 in lamprey. But the most spectacular improvement 

 of function, the most important of all the directional 

 movements in evolution has been that affecting the 

 nervous system and the sense-organs associated with 

 it. Few people who have not gone carefully into 

 the subject realize how imprisoned and windowless 

 are the existences led by lower forms of life. 



Even such physically well-organized creatures as 

 Crustacea stand at an amazingly low mental level. 

 The other day I was reading a careful account of 

 experiments on the behaviour of crabs. The method 



