castle: role of selection in evolution 



373 



4. Natural selection determines 



what classes of variations 

 shall survive and, in conse- 

 quence, what shall be the 

 variable material subjected to 

 selection in the next genera- 

 tion. 



5. The further evolution of our 



domestic animals and culti- 

 vated plants (and of man 

 himself) is to some extent 

 controllable because we can 

 by selection influence the va- 

 riability of later generations. 



4. Natural selection determines 



only what classes of varia- 

 tions shall survive, and exer- 

 cises no influence on the 

 subsequent variability of the 

 race. 



5. Evolution is beyond our con- 



trol except as we discover and 

 isolate variations. 



These two sets of contrasted view r s remind us somewhat of 

 the theological ideas of free-will and predestination respectively, 

 which resemblance will account for the preferences of some biol- 

 ogists but will not prove which is right and which is wrong. 

 This is w T holly a matter for evidence. But what conclusion one 

 reaches will depend much upon w r hat sort of evidence he studies. 

 Paleontology, geographical distribution, classification, and experi- 

 mental breeding, all present evidence which must be weighed 

 before a safe verdict can be framed. 



Paleontology, the study of the actual historical records of 

 evolution found in the rocks, indicates in the case of the most 

 complete series of fossils, as for example of the horse, the camel, 

 and the rhinoceros, that the evolution of these types was a 

 gradual process, though of course their appearance in particular 

 continents may have been abrupt, owing to migration. It 

 indicates further that these and other types, when they first 

 appeared, w r ere plastic, and generalized and varied in many 

 different ways, most of the variations later disappearing and 

 leaving only a favored few lines of specialized survivors. It 

 shows too that one variation paved the w^ay to another. The 

 five-toed horse first becomes four-toed, then three-toed, then 

 one-toed. There is no mutation from five-toed to one-toed, 

 nor from the size of a fox to that of a draft horse. As to natural 

 selection, paleontology is silent, because the causes of extinction 

 are unknow r n. But on the w T hole the weighty evidence of pale- 



