118 GENETICS 



10. PURE LINES AND NATURAL SELECTION 



From the foregoing statements it appears that by 

 means of selection within a population, such as occurs 

 normally in nature, it is not possible to get anything 

 out that was not already there to begin with. If 

 this is so, the origin of species cannot have come 

 about, as Darwin thought, through natural selection 

 by a gradual accumulation of slight favorable varia- 

 tions. The best that selection can do is to isolate 

 pure lines. Within pure lines it is quite powerless 

 to change the genotypical characters. In other 

 words, natural selection can only maintain and 

 strengthen the frontier posts that are already es- 

 tablished. It cannot break into the wilderness and 

 create new centers. 



Since the extreme members of a pure line, having 

 the same genotypical constitution, always tend to 

 backslide to mediocrity within the limits of the line 

 in question, the crucial question is : How can the 

 critical step from one genotype to another, a step 

 indispensable in the evolutionary derivation of 

 species, ever occur ? That it has repeatedly oc- 

 curred in the course of time is amply proven by the 

 fact that somehow or other we have gone from 

 Ameba to man. 



At present the only loophole of escape seems to lie 

 either in the unlikely inheritance of acquired char- 

 acters, or in mutations which make the leap from 

 one character to another, and^so eventually from one 

 type to another, without the aid of selection. 



