The Mechanism of Adaptation 331 



on to later generations by heredity, and in this way racial 

 adaptations are supposed to have originated. Thus all 

 racial or inherent adaptations are held to have come from 

 individual or acquired ones. The increased pigmentation 

 of the skin of one who is exposed to tropical light is said to 

 be inherited by his children, and so a dark-skinned race 

 arises; the stretching of the neck and legs of any animal 

 that browses on trees is supposed to be inherited, and so 

 the giraffe was evolved. Such an explanation is so simple 

 and plausible that it has been widely accepted. Unfortu- 

 nately for this attractive explanation, there is no evidence 

 that it is true. The evidences in favor of the inheritance 

 of any somatic modification are very unsatisfactory, and 

 when it comes to the inheritance of acquired adaptations, 

 critical evidence is lacking altogether. For years evidences 

 of such inheritance have been earnestly sought, but no such 

 confirmations have been found as would certainly have been 

 the case if this kind of inheritance were at all common. 



On the other hand, there seems to be no reasonable escape 

 from the postulate that modifications of the germplasm are 

 produced by environmental influences. The germ-cells, and 

 more especially the chromosomes and genes, are well pro- 

 tected from almost every change in the external environ- 

 ment, but there is an internal environment of body fluids 

 and of cytoplasmic and nuclear substances which comes into 

 much more intimate contact with the germplasm, and it 

 seems necessary to assume that certain changes in this in- 

 ternal environment may cause changes in the germplasm 

 itself. Some experimental evidence, especially that of Guyer 

 and Smith on inherited eye-defects in rabbits, favors this 

 view. 



However, such environmental modifications of the germ- 

 plasm are not generally adaptive, and the beneficial charac- 



