194 ORIGIN OF NON-SEXUAL CHARACTERS 



biologists that the thjrroid, whether that of the 

 tadpole itself or that which is supplied as food, only 

 causes the development of legs because the hereditary 

 power to develop legs is already present. The 

 question is how this hereditary power was evolved. 

 Legs are an adaptation to life on land. What we 

 have to consider and to investigate is whether the 

 legs arose as a gametic mutation or as a direct result 

 of locomotion on land. 



The general result of clinical and experimental 

 evidence is to show that the hormone of the thyroid 

 is necessary to normal development. The arrest of 

 development in cretinous children is due to some 

 deficiency of thjrroid secretion, and is counteracted 

 by the administration of thyroid extract. Excess of 

 the secretion produces a state of restlessness and 

 excitement associated with an abnormally rapid 

 rate of metabolism and protrusion of the eye-balls 

 (Graves' disease). The physiological text-books, 

 however, say nothing of precocity of development 

 in children as a result of hyperthyroidism. This, 

 however, is undoubtedly what occurs in the case of 

 tadpoles. The legs would naturally develop at some 

 time or other, after a prolonged period of larval life. 

 Feeding with thyroid causes them to develop at 

 once. I have repeated Gudernatsch's experiment 

 with the following results : — 



This year I had a considerable number of tadpoles 

 of the common English frog, which were hatched 

 between March 26 and March 29. On April 12, 

 when they had all passed the stage of external 

 gills and developed internal gills and opercula, I 

 divided them into two lots, one in a shallow pie-dish, 

 the other in a glass cylinder. To one lot I gave a 



