THEORY OF NATURAL SELECTION 245 



nourished offspring, than would the individuals which se- 

 creted 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 specialisation, 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 Ornithorhyncus, at the base of the mammalian series. 

 Through what agency the glands over a certain space be- 

 came more highly specialised than the others, I will not pre- 

 tend 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 learnt 

 to suck the breast, than in understanding how unhatched 

 chickens have learnt 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 learnt 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 ad- 

 vanced age, and afterwards 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 spe- 

 cial 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 harmlessly on each side of this elongated 

 larynx, and so safely attains the gullet behind it." Mr. Mi- 

 vart 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 form), "this at 

 least perfectly innocent and harmless structure?" It may 



