178 THE LAW OF BIRTHS AND DEATHS 



danger from an improvement in the condition of the 

 people is not over-population but de-population. Some 

 eighty years have passed since then, and Doubleday has 

 been completely justified. Yet, while the name of Malthus 

 is familiar to every social student, the name of Doubleday 

 is almost unknown. Therefore it will be no more than 

 justice to devote a few pages to a criticism of Doubleday 's 

 theory, showing its strong and weak points, and em- 

 phasising the fact that he was probably right in his main 

 contention, although his theory was ill formulated. 



Doubleday argued that when the existence of a species 

 is endangered, " a corresponding effort is invariably made 

 by Nature for its preservation and continuance by an 

 increase of fertility, and that this especially takes place 

 whenever such danger arises from a diminution of proper 

 nourishment or food, so that consequently the state of 

 depletion or the deplethoric state is favourable to fertility, 

 and that, on the other hand, the plethoric state, or state 

 of repletion, is unfavourable to fertility in the ratio of 

 the intensity of each state." 



The chief defects in this statement of the law governing 

 the variation in the degree of fertility under the influence 

 of the environment are two. In the first instance it is 

 asserted that the degree of fertility will vary in the ratio 

 of the intensity of the two states. This implies that 

 the less the amount of nourishment the greater the degree 

 of fertility. But such a result is obviously impossible. 

 According to this formula the greatest degree of fertility 

 would be reached in an absolutely barren soil, whereas 

 there could be no fertility at all in such circumstances. 

 But given a small amount of nutrition, then a certain 

 degree of fertility may appear. This will increase in a 

 direct ratio with the amount of nutrition up to the 

 point where the maximum degree of fertility is developed. 

 It is only when the amount of nutriment suitable for 



