THEORY OF NATURAL SELECTION. 211 



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 offspring, 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 specialization, 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 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 nat- 

 ural 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 un- 

 hatched chickens have learned to break the egg-shell by tap- 

 ping 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 } r oung 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 into the wind- 

 pipe. 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 harmlessly on each side 

 of this elongated larynx, and so safely attains the gullet 

 behind it." Mr. Mivart then asks, how did natural selection 

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

 on the assumption that they are descended from a marsupial 



