CHAPTER FIFTEEN 



Natural Selection 



The early post-Darwinian students of evolution were inclined to think 

 of natural selection as an all-or-none phenomenon: either an organism 

 was favorably endowed and survived to reproductive age, or it was un- 

 favorably endowed and died in the struggle for existence without leaving 

 progeny ( Figure 88 ) . Among the factors that led to the agnostic reaction 

 at the turn of the century was the fact that no such severe selection had 

 been actually observed as a general phenomenon in nature. Yet it ought 

 to be very obvious if it existed. Severe defectives were, of course, elimi- 

 nated: crippled deer cannot long escape predators. But many minor de- 

 fectives were observed in nature, and they did leave progeny. Coupled 

 with the new mutation theory of De Vries, according to which a new 

 species might be produced at a single step, these facts seemed to push 

 natural selection out of the picture entirely. 



SEVERE VERSUS MILD SELECTIVE FORCES 



But there is another possibility, not considered at the time of the agnostic 

 reaction. If a character confers only a slight disadvantage upon the organ- 



FiGURE 88. (From Simpson, 

 /. Washington Acad. Sci., V. 

 31, 1941.) 



NEGATIVE POSITIVE 



NATURAL SELECTION 



233 



