STEEILITY 193 



In other crosses the plants matured, but they developed 

 very slowly and in the end were smaller than either 

 of the parents. 



In general, therefore, it can be said that differences in 

 uniting germ plasms, when not too great, may bring about 

 both more efficient development and increased fertility. 

 Beyond that critical point of difference both fertility and 

 vigor may be decreased, but fertility is usually the hrst 

 to suffer— even complete sterility often being coupled 

 with rampant growth. Nature thus steps in before a 

 germinal heterogeneity which will endanger the health of 

 the hybrid organism has been reached, and prevents mul- 

 tiplication entirely. This is an important physiological 

 provision, since when great germinal differences exist 

 there is reduced growth as well as sterihty. Groups are 

 thus set apart which may evolve within themselves by put- 

 ting to good use heterosis and Mendelian recombination. 

 What apparently happens is this : As germinal differences 

 increase a point is reached at which the precise and com- 

 plex machinery governing gametogenesis cannot do its 

 work in the normal manner and sterility results, although 

 under the same conditions developmental cell division 

 goes on as usual. Beyond this degree of difference in the 

 uniting germ plasms, even somatic cell division is affected. 



This sterility accompanying wide crosses is an almost 

 untouched problem. We can throw no light upon it except 

 the suggestions noted in the last few sentences. For this 

 reason one may inquire why it is mentioned in this con- 

 nection at all. In spite of our comparative lack of knowl- 

 edge as to just what occurs in the cell divisions of vdde 

 crosses, however, there is an excuse for meddling. The 

 peculiar resemblance of the effect of inbreeding to the 



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