798 LIFE : OUTLINES OF GENERAL BIOLOGY 



puma is said to be only fifteen weeks. It should also be borne in 

 mind that while there is an obvious advantage in being born well- 

 equipped in body and mind, as in the foal, a similar success may 

 reward an animal born at a much less advanced stage, provided that 

 it be safely hidden away and carefully educated by its parent or 

 parents. The detailed education which the mother otter gives her 

 cubs is well known, and there are many other instances. 



Apart from the reward that comes to precocious intellectual 

 development, another advantage of prolonged gestation and 

 advanced equipment at birth may be recognised in cases where the 

 habitat is very peculiar, with unique difficulties. Thus a young 

 Cetacean must be able to swim and dive at birth, and it must also 

 be able to suckle in the water. It is not surprising then to find that 

 the gestation of porpoises and dolphins lasts for about ten months. 

 Similarly, not much is known of the intellectual gifts of the walrus, 

 but there is no doubt as to the difficulties of its boreal haunts, and 

 it is not surprising to find that its gestation lasts about a year, and 

 the lactation or nursing period for about two years. This is an 

 exceptional case that tests the rule or thesis, that prolonged gestation 

 gives the young creature a good start. 



There are, no doubt, other factors to be considered in interpreting 

 the length of ante-natal life as adaptively adjusted in the course of 

 Natural Selection, (a) Thus when there is a regularly recurrent 

 pairing time, there may have been a lengthening out, or perhaps a 

 shortening down, of the gestation so as to ensure that the young 

 ones are born at a suitable time of year. This seems to be brought 

 about in certain cases, such as the badger, by a modificational 

 adjustment, meaning bj^ this phrase that changes in the rate of 

 development are directly impressed on individuals by environ- 

 mental, nutritive, and functional peculiarities. For instance, whi^e 

 conception may occur, they say, in the badger at any time of year, 

 "the young are invariably born within a period limited to six weeks". 

 This fact, cited from Marshall's admirable Physiology of Reproduc- 

 tion, to which we are as gratefully as obviously indebted, can only 

 be explained, so far as we see, on the theory that unpropitious 

 environmental conditions bring about an arrest of development, 

 and this standstill has been actually demonstrated in the somewhat 

 extraordinary case of the badger. But apart from such difiicult 

 cases, it seems reasonable to suppose that an adaptation of length 

 of gestation to birth at appropriate seasons might be readily 

 effected, if the Jength of gestation is a variable specific character, 

 and if being born at an inappropriate time is rapidly fatal. If man 

 can by reasonable interference secure that lambs and calves and 

 foals are not born at very unfriendly times of year, there is no 

 improbability in the theory that this sort of adjustment of time of 

 birth and length of gestation to seasonal conditions may have 



