CHAP. VII.'J THEORY OF NATURAL SELECTION. 177 



cutaneous gland of its mother? And even if one was so, what 

 chance was there of the perpetuation of such a variation ? " But 

 the case is not here put fairly. It is admitted by most evolu- 

 tionists that mammals are descended from a marsupial form ; 

 and if so, the mammary glands will have been at first developed 

 within the marsupial sack. In the case of the fish (Hippocampus) 

 the eggs are hatched, and the young are reared for a time, within 

 a sack of this nature ; and an American naturalist, Mr. Lockwood, 

 believes 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 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 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 Ornitho- 

 rhyncus, at the base of the mammalian series. Through what 

 agency the glands over a certain space became more highly 

 specialised 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 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 advanced 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 special provision exist, the young one must infallibly be choked 

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



