250 THE EVOLUTION OF SEX. 



]3robably most nearly allied to tiie ordinary sebaceous type, 

 except in monotremes which appear to be divergent. Every 

 one knows that they are exclusive characteristics of mammals, 

 and are only normally functional in the female sex. Rudi- 

 mentary in the males, they may even there produce milk 

 ("witches' milk'') at birth, puberty, and under pathological 

 conditions, while cases have been put on record of men who 

 have actually given suck.* They vary greatly in position and 

 number, a large number being doubtless the primitive condi- 

 tion. In function, after the birth of offspring, the surrounding 

 tissue is specially rich in white blood-corpuscles, which probably 

 form some of the structural elements of the milk. It has also 

 been shown that the nuc'.ei of the gland cells undergo degene- 

 ration, disruption, and expulsion, and that they in all likelihood 

 form the casein elements of the nutritive fluid. 



Before birth, the mammahan embryo has been nourished 

 through the placenta, by the transfusion already referred to. 

 The alimentary canal has obviously had no experience in 

 digestive function. Before ir proceeds to digest the food of the 

 parents, it is put through a course of vdiat SoUas neatly terms 

 "gastric education,''" by feeding upon the readily assimilated 

 mother's milk. 



§ 7. Other Secretions. — Every one has heard at least of 

 "pigeon's milk," and many are familiar with its administration 

 to the young birds. This is produced by both sexes for a 

 week or so after the hatching of the young, and is the result of 

 a degeneration of the cells lining the crop. Some of the cells 

 break up, others are discharged bodily. The result forms a 

 milky emulsion-like fluid, which is regurgitated by the parents 

 into the mouth of the young bird. A similar substance is said 

 to occur in some parrots. 



Of some interest also is the supra-salivation Vv'hich occurs 

 at the breeding season in the swiftlets {Collocalia), which form 

 the edible birds' nests, the costly, though to us wofully insipid, 

 luxury of Chinese epicures. Certain salivary glands become 

 peculiarly active in these birds when breeding, and the secre- 

 tion, which, according to Green, consists chiefly of a substance 

 akin to mucin, is used to form the snow-white fibrous nest. 



Take only one other instance of peculiar secretion, curiously 



* [Merriam (Hayden's U.S. Geol. Survey, VI., p. 666) gives a definite 

 account of male lactation in Lepus bairai. 



