52 THE LAW OF BIRTHS AND DEATHS 



a warmer to a cooler climate ; in the reverse case, as 

 * when wheat was imported directly from France into 

 the West Indian Islands, it produced either wholly barren 

 spikes or furnished with one or two miserable seeds, 

 while West Indian seed by its side yielded an enormous 

 harvest.' " * 



Nothing could be more obvious than the application 

 of the principle to these facts. As the optimum point 

 for fertility will be a certain ideal combination of condi- 

 tions, therefore English, Scotch, French and Indian seed 

 will each be adapted by selection to a different combina- 

 tion of conditions. It follows that any material change 

 in these conditions in either direction can only result 

 in lessened fertility. The English farmer looks for plenty 

 of sun and rain at the right seasons, together with a rich 

 soil, to produce a heavy crop of wheat. Yet European 

 seed transplanted to the West Indies, where all these 

 things are present in excessive abundance, is rendered 

 almost completely sterile. These same varieties could 

 be adapted by careful selection to any reasonable com- 

 bination of conditions. Yet they would still remain 

 subject to the general principle which governs the varia- 

 tion of the degree of fertility in response to the direct 

 action of the environment the law of the optimum point 

 of fertility. 



Herbert Spencer, advocating the view that the degree 

 of fertility is directly proportional to the surplus of nutri- 

 tion over and above the cost of individuation, says of 

 fruit-trees : " Were it otherwise, dunging the roots of 

 a fruit-tree would be in all cases impolitic, instead of 

 being impolitic only when the growth of sexless axes is 

 still luxuriant. Were it otherwise, a tree which has 

 borne a heavy crop should, by the consequent depletion, 



1 Variation of Animals and Plants under Domestication, chap. xxiv. 



