castle: role of selection in evolution . 373 



4. Natural selection determines 4. Natural selection determines 



what classes of variations only what classes of varia- 



shall survive and, in conse- tions shall survive, and exer- 



quence, what shall be the cises no influence on the 



variable material subjected to subsequent variability of the 



selection in the next genera- race, 

 tion. 



5. The further evolution of our 5. Evolution is beyond our con- 



domestic animals and culti- trol except as we discover and 



vated plants (and of man isolate variations, 



himself) is to some extent 

 controllable because we can 

 by selection influence the va- 

 riability of later generations. 



These two sets of contrasted views 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 wholly a matter for evidence. But what conclusion one 

 reaches will depend much upon what 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, were 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 way 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 unknown. But on the whole the weighty evidence of pale- 



