1910.] The Origin of Lactation. 147 



the bladder (Weber, 1904, pp. 244, 246). However, in respect to the 

 bending of the vas deferens around the ureters the Monotremes agree rather 

 with the Placentals than with the Marsupials (AVeber, 1904, p. 244). 



When we turn to the accessory reproductive characters of Monotremes 

 especially the organs and process of lactation, we find very illuminative con- 

 ditions which suggest what may have been the history of lactation in. the 

 mammals. 



Semon (1895) gave an excellent account of the brooding habits, nature 

 of the milk, etc., of the Monotremes in his ' Zoologisches Forschungsreisen,' 

 and more briefly (1899) in his 'In the Australian Bush' (pp. 160-164). 

 Eggeling (pp. 332-340) showed that the sweat glands and the mammary 

 glands of iNIonotremes were both derived from the same undifferentiated 

 type. Bresslau in his 'Entwickelung des INIammarapparates der Mono- 

 tremen....' (1907) gave an extended description of the developmental 

 stages of the marsupium. His principal conclusions (/. c, p. 512) are: that 

 the primary " Anlagen" of the marsupium indicate that it arose in the ances- 

 tral mammals in much the same way as did the brooding ridges in birds, 

 namely, as a result of incubating the egg; and that this habit caused an in- 

 crease in the blood vessels and glands in the mammary fields, which served 

 at first to aid in keeping the egg warm. Afterward these glands gave rise 

 by a change of function to the milk glands. 



Mivart (quoted by Darwin, 1872, p. 322), in attacking the theory of 

 natural selection, raised the following objection: "Is it conceivable that the 

 young of any animal was ever saved from destruction by accidentally suck- 

 ing a drop of scarcely nutritious fluid from an accidentally hypertrophied 

 cutaneous gland of its mother?" Darwin (1872, /. c), in endeavoring to 

 meet this objection suggested that the mammary glands were at first devel- 

 oped within the marsupial sack, cited the rise of the marsupium in the fish 

 Hippocampus, and emphasised the idea that the sucking habit was "at 

 first accpiired by practice at a more advanced age, and afterward transmitted 

 to the offspring at an earlier age." 



Upon these and similar observations may be based the following general 

 hypothesis of the origin of lactation. 



(1) From the intimate developmental connection between hairs, se- 

 baceous and suboriparous glands and milk glands it may be inferred that 

 the milking habit was at first a by-product of the general process whereby 

 reptiles with variable body temperature (poecilothermous) were transformed 

 into mammals with a constant body temperature (homoeothermous). The 

 Monotremes, with an imperfectly developed homoeothermy, are equally 

 primitive in the organs and processes of lactation. 



(2) The secretion of the proto-lacteal glands at first may have served 

 to raise the temperature of the egg during incubation (Bresslau, 1907). 



