THE DEVELOPMENT OF ORGANISMS 797 



towards an increase in the time; and thus it is quite legitimate to 

 postulate variability in the length of gestation. This variability 

 might be prompted by variations in the endocrinal or hormone- 

 making system of the offspring, or in its capacity for making most 

 of the food; but it is enough to say that as regards gestation, as 

 elsewhere, variations in the rate or tempo of development are 

 common. If these variations are continued in the inheritance from 

 generation to generation, and if they are advantageous, then a 

 change in the length of the ante-natal life might become a new 

 specific character. 



Thus the question comes to be whether changes in the length of 

 gestation may be of survival value. Are they so advantageous or 

 disadvantageous that they might come to be sifted in the course of 

 Natural Selection? 



On the whole there is an advantage in economised reproductivity, 

 since it lessens the strain on the mother. But a gradual reduction 

 in the size of the family is hazardous unless there is some associated 

 variation which increases the chances of the offspring's survival. 

 This may be effected by an increased parental care, especially on 

 the mother's side, as may be illustrated by bats, which carry their 

 young one through the air. The very helpless offspring of monkeys 

 and apes is often carried about, and the mortality seems to be low. 

 Or the offspring may be hidden away, as wild cattle hide their calf 

 in the thicket, and wild deer their fawn. Or it may be through a 

 den that the safety of the family is secured, as in many Carnivores. 

 But it is plain that another way in which the safety and success 

 of the young offspring may be secured is b}' delaying its birth until 

 it is more or less able to fend for itself. A lengthening out of the 

 gestation allows the young creature to attain to a relatively 

 advanced state before it is born; and Robert Chambers had the 

 shrewd idea that a prolonged period of quiet development would 

 favour the evolution of the brain. Thus there might result an 

 improvement of intelligence and a reduction in the scope of instinct. 

 The prolonged gestation of elephants (20 months), camels (13 

 months), horses (11 months), cattle (9 months), may be referred to 

 in illustration. A prolonged quiet development of the brain, with 

 abundant nutrition but without much excitement or activity, would 

 favour cerebral advance. 



While there are many cases where prolonged gestation is associated 

 with an advanced development of the brain at birth, thus giving 

 the young creature a good send-off, as far as wits are concerned, 

 there is no doubt that the same result might be reached by having 

 the litter in some safe retreat, as in many carnivores. In such types 

 the gestation may be relatively short, e.g. four or five months 

 in the badger, which may be deceptively lengthened out in individual 

 cases by the interpolation of a period of arrest. The gestation of the 



