24 



EVOLUTION AND GENETICS 



descriptive embryology. Today it is taught in many 

 textbooks of biology. Haeckel called this interpre- 

 tation the Biogenetic Law. The parallel between the 

 historical development of antlers of deer {fig. 8) 

 and their postnatal development from year to year 

 (fig. 9) is most striking. Historically we may sup- 

 pose that the develojiment was due to the appear- 



FiG. 8. — Fossil deer antlers. The first two 

 to the left are from the niid-eosine; the third 

 is from the iip})er miocene; the fourth is from 

 the pliocene, as is also the fifth figure. The 

 figure to the right is from the "forest-bed of 

 Norfolk." (After RoTrwnes.) 



ance of hereditary variations in the germ material. 

 At the present time, the stages in their development 

 are closely correlated with age, size, and especially 

 with the yearly increase in the internal secretion of 

 the testes ; for, after castration the antlers no longer 

 develop. In living deer all the hereditary factors that 

 appeared in the past are present at each stage, but the 

 extent to which the antlers develop depends on the 

 physiological conditions mentioned above. In other 

 words, while there is a close relation in both cases 



