34 2 Adaptation to Environment 



which animals get their food and the suitable conditions 

 of preservation. Can we understand all these adapta- 

 tions, without a belief in the heredity of acquired char- 

 acters? As a matter of fact the tenacity with which 

 some authors cling to such a belief is dictated by the 

 idea that this is the only alternative to the supra- 

 naturalistic or vitalistic ideas. The writer is of the 

 opinion that we do not need to depend upon the as- 

 sumption of the heredity of acquired characters, but that 

 physiological chemistry is adequate for this purpose. 



The earlier writers explained the growth of the legs 

 in the tadpole of the frog or toad as a case of an adapta- 

 tion to life on land. We know through Gudernatsch 

 that the growth of the legs can be produced at any 

 time even in the youngest tadpole, which is unable to 

 live on the land, by feeding the animal with the thyroid 

 gland. As we have stated in Chapter VII, it is quite 

 possible that in nature the legs of the tadpole begin to 

 grow when enough of the thyroid or a similar compound 

 has been formed or is circulating in the animal. 



It might justly be claimed as a case of adaptation 

 that the egg attaches itself to the wall of the uterus 

 and calls forth the formation of the decidua. We 

 have mentioned the observation of Leo Loeb that the 

 corpus luteum of the ovary gives off a substance to the 

 blood which alters the tissues in the uterus in such a 

 way that contact with any foreign body (e. g., the egg) 

 induces this decidua formation. Again what appeared 



