OBJECTIONS TO THE THEORY OF NATURAL SELECTION 193 



lieves from what he has seen of the development of the young, that 

 they are nourished by a secretion from the cutaneous glands of the 

 sack. Now, with the early progenitors of mammals, almost before 

 they deserved to be thus designated, is it not at least possible that 

 the young might have been similarly nourished? And in this case, 

 the individuals which secreted a fluid, in some degree or manner 

 the most nutritious, so as to partake of the nature of milk, would 

 in the long-run have reared a larger number of well-nourished off- 

 spring, than would the individuals which secreted a poorer fluid; 

 and thus the cutaneous glands, which are the homologues of the 

 mammary glands, would have been improved or rendered more 

 effective. It accords with the widely extended principle of speciali- 

 zation, 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 Ornithorhynchus, at the base of the mammalian 

 series. Through what agency the glands over a certain space be- 

 came 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 selec- 

 tion, 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 egg-shell by tapping against it with their specially 

 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 helpless, half- 

 formed offspring. On this head Mr. Mivart remarks: "Did no 

 special provision exist, the young one must infallibly be choked 

 by the intrusion of the milk into the windpipe. But there is a 

 special provision. The larynx is so elongated that it rises up into 

 the posterior end of the nasal passage, and is thus enabled to give 

 free entrance to the air for the lungs, while the milk passes harm- 

 lessly on each side of this elongated larynx, and so safely attains 

 the gullet behind it." Mr. Mivart then asks, how did natural selec- 

 tion remove in the adult kangaroo (and in most other mammals, 



