54 THE LAW OF BIRTHS AND DEATHS 



sensitive to the slightest change. If the organism can 

 remain unaffected by the change, there seems no reason 

 why its reproductive system should be affected. 



On the other hand, Darwin mentions that a very slight 

 change, such as a little more or less water at some period 

 of growth, will sometimes determine fertility or sterility. 

 This, again, is in no way inconsistent with the principle. 

 A very slight cause may have a very powerful effect on 

 the vitality of the organism as a whole, and thus affect 

 the degree of fertility. We shall see later good reason 

 to believe that a little more or less moisture in the atmos- 

 phere may, by its direct effect on the individual, and by 

 its indirect effects through its influence on social habits, 

 powerfully affect the fertility of the human race. 



These few remarks on the influence of the environment 

 on the degree of vegetable fertility are little more than 

 a digression from our main purpose. But they have 

 served to complete the statement of the principle and 

 to show that it applies to vegetable as well as to animal 

 fertility ; while that the same general law runs through 

 the whole scheme of organic reproduction will be shown 

 as we proceed. In considering the facts it should be 

 borne in mind that the degree of sexual fertility must 

 be judged by the number of fertile seeds, not by the 

 amount of fruit. It should also be borne in mind that 

 we cannot expect from a general principle a detailed 

 explanation of particular cases, or an apparent conformity 

 in every case. Within the scope of every general principle 

 there are at work numbers of minor and counteracting 

 principles. These often obscure the action of the main 

 principle and produce results at first sight contradictory 

 or inconsistent. 



The attitude of many people towards a general principle 

 may be illustrated thus : Let us suppose ourselves stand- 



