202 THE DESCENT OF MAX. [Part L 



male fislies hatch the eggs witliin their mouths or bran- 

 chial cavities ; that certain male toads take the chaplets 

 of eggs from the females and wind them round their own 

 thighs, keeping them there until the tad2:»oles are born ; 

 that certain male birds undertake the whole duty of incu- 

 bation, and that male pigeons, as well as the females, feed 

 their nestlings with a secretion from their crops. But the 

 above suspicion first occurred to me from the mammary 

 glands in male mammals being developed so much more 

 perfectly than the rudiments of those other accessory re- 

 productive parts, which are found in the one sex though 

 proper to the otlier. The mammary glands and nipples, 

 as they exist in male mammals, can indeed hardly be 

 called rudimentary ; they are sim])ly not fully developed 

 and not functionally active. They are sympathetically 

 affected under the influence of certain diseases, like the 

 same oi'gans in the female. At birth they often secrete a 

 few drops of milk ; and they have been known occasion- 

 ally in man and other mammals to become well devel- 

 oped, and to yield a fair supply of milk. iSTow if we sup- 

 pose that during a former prolonged period male mam- 

 mals aided the females in nursing their offspring, and that 

 afterward from some cause, as from a smaller number of 

 young being produced, the males ceased giving this aid, 

 disuse of the organs during maturity would lead to their 

 becoming inactive ; and from two well-known principles 

 of inheritance this state of inactivity would probably be 

 transmitted to the males at the corresponding age of ma- 

 turity. But at all earlier ages these organs would be left 

 unaffected, so that they would be equally well developed 

 in the young of both sexes. 



mouths, see a very iatcresthig paper by Prof. Wyman, in ' Proc. Boston 

 Soc. of Nat. Hist.' Sept. 15, 1857; also Prof. Turner, in 'Journal of 

 Anat. and Phys.' Nov. 1, 1800, p. 78. Dr. Giintherhas likewise described 

 similar cases. 



