INTRODUCTION xi 



claimed. One of man's earliest ancestors was almost 

 certainly a unicellular animal. A little later he very 

 probably passed through a gastraea stage. He trav- 

 ersed fish, amphibian, and reptilian grades. The 

 oviparous monotrenie and the marsupial almost cer- 

 tainly represent lower mammalian ancestral stages. 

 But what kind of fish, what species of amphibian, 

 what form of reptiles most closely resembles the old 

 ancestor ? How did each of these ancestors look ? I 

 do not know. It looks as if our ancestral tree were 

 entirely uncertain and we were left without any founda- 

 tion for history or argument. 



But the history of the development of anatomical 

 details, however important and desirable, is not the 

 only history which can be written, nor is it essential. 

 It would be interesting to know the size of brain, girth 

 of chest, average stature, and the features of the an- 

 cient Greeks and Romans. But this is not the most 

 important part of their history, nor is it essential. The 

 great question is, What did they contribute to human 

 progress ? 



Even if we cannot accurately portray the anatomical 

 details of a single ancestral stage, can we perhaps dis- 

 cover what function governed its life and was the aim 

 of its existence ? Did it live to eat, or to move, or to 

 think? If we cannot tell exactly how it looked, can 

 we tell what it lived for and what it contributed to the 

 evolution of man ? 



Now, the sequence of dominant functions or aims in 

 life can be traced with far more ease and safety, not 

 to say certainty, than one of anatomical details. The 

 latter characterize small groups, genera, families, or 

 classes ; while the dominant function characterizes all 



