86 THE GENETIC AND THE OPERATIVE EVIDENCE 



acting directly on the " metabolism," cause the changes observed, then 

 the experiments mean that environmental conditions affect directly 

 the development of the nuptial and the eclipse plumage; but if, as I 

 suggest here, the effects observed are due directly to the environmental 

 action through its effects on the testes, then the results fall more nearly 

 into line with those of Goodale on ducks, etc. 



C. EVIDENCE FROM AMPHIBIA. 



The thumbs of frogs enlarge at the breeding-season and shrink 

 afterwards. The enlarged thumb is used by the male in clasping the 

 female during copulation, and the rough papillse that appear over its 

 surface at this time may also help to anchor the male in his precarious 

 position on the back of the female. Since the pads and their papillae 

 are used in copulation, they belong rather in the class of accessory 

 organs of reproduction than in the class of secondary sexual characters. 

 Smith and Schuster state for Rana fusca that the testes are at their 

 smallest size in March and April after the breeding-season. From that 

 tune until August they steadily increase in size and reach their maxi- 

 mum size in September. From September to March they are inactive 

 and full size, until the shedding of the sperm in March brings them 

 soon afterward to their lowest point again. It is to be noted that the 

 increase after March is associated with the increase in division rate of 

 the spermatogonia. The ripening of the sperm is finished in October. 



The thumb-pads with their pigmented papillse are "cast off" 

 immediately after the breeding-season, the thumb remaining smooth 

 from May to September. The reduction of the pad is usually due to 

 the reduction of the glands and the disappearance of the papillse. 

 Smith and Schuster state: "During the months when the most active 

 growth of the testis is taking place the thumb-pads remain inactive 

 and smooth." The implication, apparently, is that one ought to expect 

 the growth in the thumb to take place when the germ-cells are most 

 actively dividing, if its growth is connected with then* activity; but 

 there are no grounds for such expectations, because the influence of the 

 gonad may have nothing to do with the division rate of the germ-cells, 

 but rather with interstitial or other cells, and even here less with their 

 division rate than with their period of greater secretive activity. 



"In August and September the epidermal papillae begin to be obvious, and 

 from this time onwards until about February a continuous increase of the 

 epidermal papillae and pigmentation occurs. During the greater part of this 

 time, when the thumb-pads are attaining their characteristic rough and pig- 

 mented appearance, the testes remain inactive and unchanged a fact which 

 has been too readily overlooked by writers on the correlation of the primary 

 and secondary sexual characters." 



Nussbaum (1909) and later Meisenheimer (1911) found that after 

 castration the thumb-pads disappear. Smith confirms this report in 

 all essential respects, although in certain details concerning the papillae 



