328 THE STUDY OF ANIMAL LIFE CHAP. 



there are also considerable risks of the parents being 

 caught by enemies or of the eggs being discovered. It is 

 obviously advantageous that the period of embryonic 

 development should be condensed, and that is what has 

 happened. The very warmth of incubation quickens the 

 rate of development. The cock that crows in the morn 

 is, on the whole, except in brain, as complex a creature 

 as the cow that tosses the dog, but the chick breaks its 

 way out of the egg-shell after three weeks of incubation, 

 whereas the calf is carried by the cow for about nine months 

 of ante-natal life. Mammals usually illustrate a lengthen- 

 ing out of the embryonic period and birds a shortening 

 of it. 



Whenever it is of great importance that the young 

 mammal should be able to move about and fend for 

 itself soon after birth, there is a prolongation of the period 

 of gestation and the } r oung one is born at an advanced 

 stage of development. It would be easy to find a Shet- 

 land pony of the same size and weight as a kangaroo, 

 but whereas the mother kangaroo carries her young forty 

 days before birth, the pony mare has a gestation of eleven 

 months. Part of the interpretation is that the pony is 

 a much higher mammal than a kangaroo, and has therefore 

 a slower and longer development ; but the other part of 

 the interpretation is that in natural conditions the foal 

 has to run beside the mare very soon after birth, and 

 must therefore be born well-equipped, while the young 

 kangaroo is stowed away in the maternal marsupium, 

 where, unable even to suck, it has the milk injected 

 into its mouth by a special muscular arrangement on 

 the mother's part. 



In whales, which are very highly specialised mammals, 

 there is usually but a single calf, and it is carried before 

 birth for about a year. This may be interpreted in 

 connection with the intricacy of the result, for a whale 

 ought to take longer to develop than a rat, and we sec 

 the same thing in the highly specialised elephant, which 

 has a gestation of twenty-two months. But in both 

 cases the rest of the interpretation is that the conditions 

 of life are such that the only feasible kind of offspring is 



