BASIS OF THE PKOBLEM 61 



would be increased, and starvation or injurious semi-starvation 

 would result. Any considerable increase in the strength of 

 fecundity beyond that which is essential could not therefore be 

 beneficial. 1 



The facts regarding the conditions of life and the dangers to 

 which the young of different species are exposed coincide with 

 the view that the strength of reproduction is in the main deter- 

 mined by the sum of these dangers. The greater strength of 

 fecundity among those species which shed their eggs into the 

 water cannot be attributed, as we have seen, to any failure of 

 fertilization on a large scale ; it may be attributed, however, 

 to the numerous dangers which the young of such species must 

 encounter. The chance that any single fertilized egg will grow 

 into an adult is far less in these cases than w r hen the eggs are 

 retained in the body of the mother and when the young are 

 guarded by the parent, and, unless fecundity was on a large 

 scale, a sufficient number would not survive. Among plants 

 reproduction must always be on a large scale because it is neces- 

 sary to ensure that a sufficient number of seeds will fall, not only 

 on suitable ground, but on suitable ground that is not so occupied 

 as to prevent growth to the adult stage, in addition to the necessity 

 of providing such numbers as will ensure a sufficient number 

 passing through all the other dangers. Eggs and young aban- 

 doned by the parents may be variously exposed to danger. The 

 development of instincts common among insects which lead to 

 the hiding of eggs in places where danger of destruction at the 

 hand of enemies is decreased, and to the provision of food for the 

 young when they develop from the eggs, is accompanied by 

 a decrease in fecundity. Further developments of the parental 

 instinct are accompanied by decreases in fecundity as the 

 degree of danger lessens. Parental care for the young is not 

 uncommon among the invertebrates. There are a number of 

 cases in which the young are retained in brood-pouches, as for 

 instance by the common water-flea. The fresh-water leech, 

 Clepsine, broods over its young. Among the vertebrates the fish 



1 For the sake of simplicity fecundity has been spoken of as though it was 

 fixed at a certain strength for each species. As a matter of fact it varies within 

 fairly wide limits increasing with better conditions. In this fact lies the explana- 

 tion of the increase of species under favourable conditions which has often been 

 observed, although, when conditions are less favourable, there is little or no 

 evidence of starvation among such species. 



