INDIVIDUALS AND SPECIES 37 



constituents of the food supply of a vast majority 

 of living things. Petrunkevitch ^° calls attention 

 to the fact that a number of parasitic worms, in- 

 cluding Taenia solium, while they must tolerate a 

 considerable variation of chemical environment 

 in their normal hosts, cannot be raised in other 

 animals. And it is commonly known that con- 

 genital thyroid deficiency so interferes with normal 

 development that cretinism results. 



When we examine some of the discoveries of 

 experimental embryology a certain independence 

 of the heritage also becomes evident. At the outset 

 of development in sexual organisms, with the ex- 

 ception of a few cases of natural parthenogenesis, 

 union of the ovum with a spermatozoon is a neces- 

 sary preface to the formation of a new individual. 

 But Loeb ^^ found many years ago that a slight 

 increase in salt concentration in sea water was 

 adequate to produce artificial parthenogenesis in 

 the eggs of the sea urchin, development continuing 

 to the production of swimming larvae, and in 1916 

 he wrote that he had "seven parthenogenetic 

 frogs over a year old, produced by merely punctur- 

 ing the eggs with a fine needle." These frogs are 

 said to have been as large as normal frogs of the 

 same age and in no way different from frogs pro- 

 duced in the natural way. 



" Organic Adaptation^ 1924. 



^^ The Mechanistic Conception of Lijct p. 7, 1912. 



