INDIVIDUALS AND SPECIES 39 



other than the usual nerves, of the same things 

 which would normally enable them to complete 

 their development. Bones, muscles, and nerves 

 are not peculiar to any one part of the body, hence 

 the substances required for their development are 

 everywhere present. Those features of their de- 

 velopment which are associated with a given loca- 

 tion, however, may be abnormal in transplanted 

 limbs; they receive an abnormal innervation, if 

 any, and have abnormal external relationships, 

 hence they have accomplished only a part of the 

 normal course of development. And it is worthy 

 of note that a relatively small percentage of the 

 transplanted limb buds are free from conspicuous 

 abnormality. 



We may accept as fact the conclusion that 

 everything which appears in the organism is a 

 product of definite interacting factors in the herit- 

 age and in the environment, and if either factor 

 departs from the normal, we may expect the result 

 likewise to depart from the normal. 



Since the individuals thus brought into existence 

 constitute species, it is an accurate conclusion also 

 to say that the species is the result of interacting 

 heritage and environment, but here additional com- 

 plexity enters. Taxonomically we may accept the 

 idea of a "mental cross-section of a line of evolu- 

 tion" and so limit our species to a fixed time level 

 in the present or to definite geological horizons 



