190 Miscellaneous Objections to the Chap, vil 



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- 

 sation, -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 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 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 elon- 

 gated 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 form), « this at least perfectly innocent 

 and harmlessstructure?" It may be suggested in answer that the 

 voice which is certainly of high importance to many animals, could 

 hardly have been used with full force as long as the larynx entered 

 the nasal passage ; and Professor Flower has suggested to me that 



™7 T, 1 W ° Uld haVG Srea% interfered with an a ^al swallow- 

 mg solid food. 



