240 METAMORPHOSIS AND 



the diagnostic characters of many larger divisions in 

 classification. 



3. Some of the most striking cases of adaptation, 

 such as the organs of respiration and circulation 

 in terrestrial Vertebrates, and the asj^mmetry of 

 Flat-fishes, are developed in the individual by a 

 metamorphosis which is generally regarded as a 

 recapitulation of the ancestral evolution. No cases 

 of mutation or gametogenic variation hitherto 

 described exhibit a similar metamorphosis or re- 

 capitulation. 



4. Secondary sexual characters, usually in the 

 male sex, correspond in their development with the 

 development of maturity and functional activity in 

 the gonads, and it has been proved that the latter 

 influence the former by means of ' hormones ' or 

 internal secretions. The evidence concerning sex 

 and sex-linked characters and the localisation of 

 their factors in the chromosomes of the gametes has 

 no bearing on the action of hormones. 



5. The facts concerning the action of hormones 

 are beyond the scope of current conceptions of the 

 action of factors or genes localised in the gametes 

 and particularly in the chromosomes. According 

 to these conceptions, characters are determined 

 entirely by the genes in the chromosomes, whereas in 

 certain cases the development of organs or characters 

 depends on a chemical substance secreted in some 

 distant part of the body. 



6. It was formerly stated that no process was 

 known or could be conceived by which modifications 

 produced in the soma by external stimuli could 

 affect the determinants in the gametes in such a way 

 that the modifications would be inherited. The 



