46 THE LAW OF BIRTHS AND DEATHS 



inheritance and must be adjusted to a certain ideal 

 combination of conditions, it follows that any variation 

 from this ideal in any direction must result in lessened 

 fertility. 



The potential degree of fertility being inherited and 

 subject to variation, it follows that those domesticated 

 plants which are cultivated for a heavy yield of seed 

 will be much more fertile than similar wild types. The 

 heavy yield of seed under favourable conditions will be 

 produced by selection. 



Darwin, it is true, expresses a somewhat different 

 opinion : " With those plants which are valued for their 

 seed, the direct selection of the more fertile individuals 

 has probably much increased their fertility ; and in all 

 cases this may have occurred indirectly, from the better 

 chance of some of the numerous offspring of the more 

 fertile individuals having been preserved. But . . . with 

 plants like carrots, cabbages and asparagus, which are 

 not valued for their prolificacy, selection can have played 

 only a subordinate part, and their increased fertility 

 must be attributed to the more favourable conditions 

 of life under which they have long existed." 1 



This last sentence is an example of the explanation 

 which explains nothing. Why should a long continuance 

 of favourable conditions have produced a high degree 

 of reproductive fertility in some plants, whilst, as we 

 shall presently see, it has produced in others almost 

 complete sterility ? It is notorious that all those plants 

 which are valued for their seed crop, such as corn, peas, 

 beans, etc., have reached their present high degree of 

 fertility through long and careful selection. But what 

 of those plants which are not valued for their seed ? 

 These plants may be divided into two classes : those which 



1 Variation oj Animals and Plants under Domestication, chap, xviii. 



