6 7 6 WHALES xxvi 



whale are not clear. Whales have been abundant throughout the 

 Tertiary, but there is not yet sufficient evidence available to recon- 

 struct their full phylogenetic history. In broad outlines, however, it 

 is clear that there has been a progressive adoption of features suitable 

 for aquatic life, the long-bodied, heterodont basilosaurs giving place 

 to the shorter, stream-lined modern whales, provided with suitable 

 stabilizing fins and with the mouth highly specialized for eating fish, 

 cephalopods, or plankton. It is impossible to say whether the changing 

 of the populations is due to indirect influences of climatic changes or 

 to factors within the animal populations themselves. The course of 

 whale evolution, like that of teleosts, appears to have produced increas- 

 ing efficiency within a single habitat, rather than a progressive coloniza- 

 tion of new fields; but this appearance may be only a reflection of our 

 ignorance and lack of knowledge of the varied and changing condition 

 of the sea. 



