406 EVOLUTION AND GENETICS 



at random during reproduction and so will have an effect on the 

 evolution of the species. Certainly these organs can have no 

 effect in determining the survivors of a generation or the propaga- 

 tion of the species, and so they cannot be the basis for natural 

 selection. It is equally certain that they will be inherited, regard- 

 less of their state of development; the greatly reduced will be 

 mingled with the highly developed and the result will be an aver- 

 aging of development of the organ in the entire species. This is, 

 in effect, a reduction in the case of formerly useful organs, but 

 it can hardly luring about an extreme degree of modification. 



In 1904 Weismann wrote of this theory: "... I was obliged 

 to seek for some other factor in modification, which should be 

 sufficient to effect the degeneration of a disused part, and for a 

 time I thought I had found this in panmixia, that is, in the mingling 

 of all together, well and less well equipped alike. This factor 

 does certainly operate, but the more I thought over it the clearer 

 it became to me that there must be some other factor at work as 

 well, for while panmixia might explain the deterioration of an 

 organ, it could not explain its decrease in size, its gradual wearing 

 away, and ultimate total disappearance. Yet this is the path 

 followed, slowly indeed, but quite surely, by all organs which 

 have become useless." We now recognize this process, which is 

 nothing more than the amphimixis of modern biology, as a means 

 of diversification, or distribution of the various degrees of variation 

 of specific characters, and of maintaining a reasonably definite 

 association of such characters. 



Germinal Selection. Weismann's realization of the weakness 

 of the theory of panmixia led him to formulate under the name of 

 germinal selection a " theory which now seems of no more value 

 than the former, yet it was based on some conceptions of living 

 substance which are worthy of consideration. An examination of 

 these conceptions leads one to the feeling that Weismann very 

 narrowly missed some important conclusions. In judging his 

 work we must remember that biology was scarcely beyond its 

 infancy during the early years of his scientific activity, and that 

 the great modern discoveries in genetics were little more than 

 begun when he died in 1914, at the age of eighty years. 



Weismann assumed as a starting point for his theory that the 

 germ plasm is composed of different living units or "determinants" 

 which control the development of different parts of the organism. 



