228 THE BIOLOGY OF BIRDS 



masculine characters in old age, or when the ovary is diseased 

 or injured. In modern physiological language, the hormone 

 that inhibits masculinity is no longer operative. Con- 

 versely, but more rarely, a castrated male may exhibit 

 feminine characters. Thus Darwin noted that a capon may 

 incubate eggs and bring up the young, and that sterile male 

 hybrids between the pheasant and the fowl may behave in a 

 similar way (see also Marshall, 1910, 315 and 654). 



To the question why the fact that the fertilised ovum is 

 going to develop into a male (or a female) should ipso facto 

 imply that all the masculine (or the feminine) characters are 

 to find expression, we have given the answer that the 

 characters are all correlated, they are there or not there en 

 hloc, they form a sex-linked assemblance. And as a reason 

 for this correlation we have suggested (i) that all masculine 

 (or feminine) characters originally arose as germinal varia- 

 tions in gametes predisposed to develop into males (or 

 females), and (2) that in some cases these variations may be 

 plausibly interpreted as congruent or solidary with the 

 characteristic male (or female) diathesis. And to this there 

 requires to be added the very important consideration that 

 just as a thyroid gland and a pituitary gland have arisen in 

 the course of evolution with most important functions in the 

 internal economy of the organism, so in the course of evolu- 

 tion the gonadial glands have arisen, whose internal secre- 

 tions, working in harmony with other internal secretions, 

 serve as the liberating stimuli and indirectly as the regulators 

 of the development of the sex-characters. 



§ 12. Hormones and Sex- Characters 



Modern work has shown that the development of 

 secondary sex-characters has sometimes become linked to the 

 reproductive organs, from which there come internal 

 secretions with stimulating or inhibiting potency. These 

 internal secretions (hormones which excite and chalones 

 which slow down) are carried by the blood from the repro- 



