248 REV. J. T. aULICK ON DTYERGENT EYOLTJTION 



successive generations. Among those that would be equally pro- 

 ductive if equally nourished, the ratio of propagation varies 

 directly as the degree of sustentation above a certain minimum 

 (and perhaps belovr a certain maximum), and therefore directly 

 as the degree of adaptation that secures this sustentation. This 

 propagation according to degrees of adaptation to the environment 

 is what I understand hy natural selection. But among those 

 that are equally adapted to the environment the ratio of propa- 

 gation varies directly as the ratio of fertilit5^ This propagation 

 according to degrees of fertility is what I call the Law of Cumu- 

 lative Fertility. It is not due to different degrees of success, or 

 to any advantage which the individuals of one form have over 

 those of other forms ; but simply to the higher ratio of multipli- 

 cation in the more fertile forms securing the intergeneratiou of 

 the more fertile. In connection with natural selection it ensures, 

 in the descendants, the predominance of the better adapted of the 

 more fertile, and the more fertile of the better adapted. 



At the close of the previous chapter I called attention to the 

 fact that innumerable Local Segregations and other imperfect 

 forms of Segeneration are being constantly broken down, partly 

 by the increase of numbers and partly by the superior fertility 

 and vigour of offspring produced by crossing. It seems to be a 

 fundamental law that vigour and variation in the offspring depend 

 on some degree of diversity of constitution in the parents, and 

 diversity of constitution that is not entirely fluctuating depends 

 on some degree of Positive Segregation; therefore vigour and 

 variation depend on the breaking-down of incipient Segrega- 

 tions, and on the interfusion of the slightly divergent forms 

 that had been partially segregated. But in the history of 

 every race that is winning success by its vigour and varia- 

 tion there is liable to come a time M^ien some variety, inher- 

 iting sufficient vigour to sustain itself, even if limited to the 

 benefits of crossing with the individuals of the same variety, 

 becomes partially Segregated. As we have already seen. Segre- 

 gation, in so far as it depends on the qualities of the organism, 

 tends ever to become more and more intense ; but, in the very 

 nature of things, not only will the Segregation be for many 

 generations only partial, but partial Segregation, though it may 

 greatly delay the submerging of different groups in one common 

 group, w^ill never prevent that result being finally reached. 

 Though the siphon that connects two tanks of water be ever so 



