SEX-CHARACTERS IN EVOLUTION 127 



from which the antlers arise are formed, each of these 

 cells must be supposed to contain the same chromo- 

 somes as the original ovum from which the cells have 

 descended by repeated cell-division. The factors in 

 these chromosomes corresponding to the forehead 

 have been stimulated while in the parent animal 

 by hormones from the outgrowth of tissue produced 

 by external mechanical stimulation, while at the 

 same time they were permeated by the testicular 

 hormone produced either by the gametocytes them- 

 selves or by interstitial cells of the testis. When the 

 head begins to form in the process of individual 

 development, the factors, according to my theory, 

 have a tendency to form the special growth of tissue 

 of which the incipient antler consists, but part of 

 the stimulus is wanting, and is not completed until 

 the testicular hormone is produced and diffused into 

 the circulation — that is to say, when the testes are 

 becoming mature and functional. 



I do not claim that this theory is complete — it is 

 impossible to understand the process completely in 

 the present state of loiowledge — but I maintain that 

 it is the only theory which affords any explanation 

 of the remarkable facts concerning the influence of 

 the hormones from the reproductive organs on the 

 development of secondary sexual characters, while 

 at the same time explaining the adaptive relation of 

 these characters or organs to the sexual habits of the 

 various species. On the mutation hypothesis, adapta- 

 tion is purely accidental. T. H. Morgan considers 

 that the appearance of two slightly different shades of 

 eye colour in male and female in a culture of a fruit- 

 fly in a bottle is sufficient to settle the whole problem 

 of sexual dimorphism, and to supersede Darwin's 



