EVOLUTION 19 



greater or lesser degrees of divergence of animals and 

 plants from one or more common roots, and have sought 

 to trace their evolutionary affinity. 



At the lowest level of variation— that is, within a spe- 

 cies—a population of animals is distinguished by such 

 minor differences that they are given no classificatory 

 significance by the biologist, and may present no advan- 

 tage or disadvantage biologically— as, for example, varia- 

 tion in the numbers of scales and their arrangement, in 

 coloration or in size, or in breeding or feeding habits. 

 What does it matter if a fish has 12, 14, or 18 rays in 

 the dorsal fin except to afford the species sphtter the 

 dehght of naming three species instead of one? But if 

 geographic isolation prevents back-breeding, and muta- 

 tion continues to increase the number of spines in one 

 stock and to decrease it in another, the process of varia- 

 tion may lead to two forms that rarely or never inter- 

 breed and the biologist will be justffied in dividing them 

 into two distinct species. The evolution of such a 'sub- 

 species' within a species is often the beginning of the 

 evolution of new species. 



With more extreme variation the population as a 

 whole changes its character to such an extent that spe- 

 cies become wholly different, giving rise to 'genera,' 'sub- 

 famihes,' and 'families.' Here adaptation becomes more 

 obvious, the randomness of variation having been sub- 

 merged or obhterated by selection so that evolution be- 

 gins to take on the appearance of sustained direction. 

 With further variation and selection, new forms appear 

 that are now so unhke their predecessors as to give rise 

 to classificatory units of still higher rank, such as 'orders' 

 and 'classes.' There is no evidence, however, that even 

 these major steps involve either a sudden, stepwise trans- 

 formation or an acceleration of the mutation rate as it is 

 observed within a species. Rather they are the product 

 of the long accumulation of small, random mutations 

 operating in such a direction as to lead to what may be 

 a distinct change in the mode of hfe. Here is what the 



