62 BASIS OF THE PEOBLEM 



for the most part take no care of their young. Some fish, however, 

 such as Gasterosteus, the Stickleback, make nests. No great 

 advance is found among the Amphibia or among the reptiles. 

 One reptilian group, however, the Chelonia, shows a remarkable 

 advance ; they live in pairs and guard their young with care. If 

 marriage be defined as ' a more or less durable connexion between 

 male and female lasting beyond the mere act of propagation till 

 after the birth of the offspring ',^ then the beginning of marriage 

 is discernable in this group. This more or less durable connexion 

 between male and female is a well-known characteristic of birds, 

 as is also care for the young. ' Most birds when they pair do so 

 for good and all until one or the other dies.' ^ The connexion is 

 not so durable, and the parental instinct is not so highly developed, 

 among mammals as among birds. The retention of the young 

 within the body of the mammalian mother, however, greatly 

 decreases the dangers to which the young are exposed. It is also 

 worthy of note that those species both of birds and mammals 

 which prey upon others are on the whole less fecund than other 

 species, a fact which is to be connected with the lower degree of 

 danger to which their offspring are exposed. 



The lower degree of danger to which the young are exposed, 

 the less the fecundity ; and the less the fecundity, provided that 

 it reaches the strength necessary to preserve the species, the 

 better on the whole for the species. In this manner we reach 

 the second generalization regarding the quantitative aspect of 

 the population problem among species in a state of nature, which 

 may be stated by saying that the strength of fecundity in any 

 species is determined by the sum of all the dangers to which the 

 young of that species are exposed. This must be qualified, in so 

 far as fecundity and fertility are not the same, by adding to the 

 dangers to which the young are exposed the danger that a certain 

 proportion of eggs will not be fertilized. It follows that among 

 men, since fecundity and fertihty are not the same for quite 

 other than ' mechanical ' reasons, fecundity is not directly related 

 to the dangers to which the young are exposed. It is clear that, 

 when men, as is now the case on a large scale, both abstain from 

 intercourse and interfere with the natural result of intercourse, 

 and at the same time increase in number, the strength of fecundity 



* Westermarck, History oj Human Marriage, p. 19. * Brehm, Bird Life, 



p. 285. 



