CHAPTER VIII 



Experimental biology continued. Influence of environment 

 in determining the development of organisms. Effects of 

 temperature, light, moisture, chemicals and food upon the 

 form of ammals and plants. The control of sex. 



And what of the development of this marvelous cell ? What 

 hand guides the growth of the future organism in all its 

 wonderful detail from this apparently simple, but unspeakably 

 complex drop of protoplasm? Is it predestined, or is it 

 plastic, to be molded by the experimenter at his will? These 

 two possibilities are in no wise incompatible with each other. 

 While the pattern of the cloth may be fixed, the form of the 

 garment to be shaped therefrom is as variable as the caprice 

 of fashion. Nor can the former be permanently fixed, unless 

 evolution is a myth. 



As we have already seen in Mendelian inheritance the char- 

 acter determiners are apparently distributed according to the 

 laws of chance. But there is evidence showing that this dis- 

 tribution can, to some extent at least, be controlled. 



If the female fruit fly be exposed to temperatures ranging 

 from 50 to 86 C. at a certain period during the maturation 

 of her germ cells, it is found that the amount of crossing 

 over between two factors may be increased by more than 

 100%. That external factors may profoundly influence 

 Mendelian results has been clearly shown by Tower. The 

 genus Leptinotarsa, of which the common potato beetle is a 

 member, shows several color variations, which have been 

 designated as species. When the female L. signaticollis is 

 crossed with the male L. diversa at an average temperature 

 of 75-79 F. and a relative humidity of 75%, the hybrid 

 offspring fall into two distinct groups of practically equal 

 numbers, one of them indistinguishable from the mother and 

 the other intermediate between the two parents. The former 

 group breeds true for several generations ; but the latter when 

 inbred give the typical Mendelian ratio of 1 signaticollis, 

 2 intermediate and 1 diversa, the first and last of which breed 

 true, but the second continues to split up in further breeding, 

 into the two parent and the intermediate types. If however 

 the crossing be done at an average temperature of 50 to 75 



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