THEORY OF NATURAL SELECTION. 225 



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 ef ects of use, or of 

 natural selection. 



The development of the mammary glands would have 

 been of no service, and could not have been affected 

 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 learned to suck the breast, than in under- 

 standing 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 after- 

 ward 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 in- 

 trusion 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 assump- 

 tion that they are descended from a marsupial form), "this 

 at least perfectly .innocent and harmless structure?" It 

 may be suggested in answer that the voice, which is cer- 

 tainly of high importance to many animals, could hardly 

 have been used with full force as long as the larynx en- 

 tered the nasal passage; and Professor Flower has suggested 

 to me that this structure would have greatly interfered with 

 an animal swallowing solid food. 



We will now turn for a short space to the lower divisions 

 of the animal kingdom. The Echitiodermata (star-fishes, 

 sea-urchins, etc.) are furnished with remarkable organs, 

 called pe"dicellari£e, which consist, when well developed, of 



