20 ESSAYS OF A BIOLOGIST 



history in large measure the history of the increase 

 in size of social units ? 



But size alone is not enough ; there is also a definite 

 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 necessary 

 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. Pro- 

 tection 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 im- 

 portant 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 



