OBJECTIONS TO THE THEORY 323 



tion, that the glands over a certain space of the sack 

 should have become more highly developed than the 

 remainder; and they would then have formed a breast, 

 but at first without a nipple, as we see in the Ornitho- 

 rhynchus, at the base of the mammalian series. Through 

 what agency the glands over a certain space became more 

 highly specialized than the others, I will not pretend to 

 decide, whether in part through compensation of growth, 

 the effects of use, or of natural selection. 



The development of the mammary glands would have 

 been of no service, and could not have been effected 

 through natural selection, unless the young at the same 

 time were able to partake of the secretion. There is no 

 greater difficulty in understanding how young mammals 

 have instinctively learned to suck the breast than in 

 understanding how unhatched chickens have learned to 

 break the eggshell by tapping against it with their spe- 

 cially adapted beaks; or how, a few hours after leaving 

 the shell, they have learned to pick up grains of food. 

 In such cases the most probable solution seems to be, 

 that the habit was at first acquired by practice at a more 

 advanced age, and afterward transmitted to the offspring 

 at an earlier age. But the young kangaroo is said not to 

 suck, only to cling to the nipple of its mother, who has 

 the power of injecting milk into the mouth of her help- 

 less, half-formed offspring. On this head Mr. Mivart re- 

 marks: "Did no special provision exist, the young one 

 must infallibly be choked by the intrusion of the milk 

 linto the windpipe. But there is a special provision. The 

 [larynx is so elongated that it rises up into the posterior 

 lend of the nasal passage, and is thus enabled to give free 

 jntrance to the air for the lungs, while the milk passes 



