90 ANALYSIS OF THE FOUR PRINCIPLES. 
between types when their sexual elements are incapable of uniting in 
fertilized germs under any conditions. Prepotential isolation exists 
when cross-fertilization is possible if the alien fertilizing element has 
the advantage of being applied some time in advance; but if the fer- 
tilizing element of the same variety or species is applied at the same 
time, or in some cases at any time during several hours that follow, 
mixed fertilization is prevented by the prepotence of the pure fertiliz- 
ing element. 
10. Fecundal Selection. 
Fecundal selection secures coordination between the number of ova 
and the supply of fertilizing elements required for the fertilizing of the 
same in connection with methods used to secure the bringing of these 
elements to the ova. The codrdination between the number of fer- 
tilized ova and the power of the parents or community for production, 
rearing, and training of offspring is secured by what I call filio-parental 
selection. The combined action of these two principles tends to 
bring the standard of fertility for the group up to the highest point 
that is permitted by the average capacity of the parents for producing, 
rearing, and training the offspring. Inmy paper on Divergent Evolu- 
tion, read before the Linnean Society in 1887, after referring to the 
principles of segregate fecundity and segregate vigor, I made the fol- 
lowing statement concerning ‘‘The Nature of Cumulative Fertility”: 
* * * Fertility increases through the breeding together of the more fertile 
resulting from the fact that more than half of each generation are the offspring of 
parents of more than average fertility. As the breeding together of the more vig- 
orous and the better adapted, caused by their superior success, tends to increase 
the vigor and adaptation of successive generations, * * * so the breeding 
together of the more fertile, caused by the larger proportion of offspring produced 
by the more fertile, tends to increase the fertility of successive generations. Among 
those that would be equally productive if equally nourished, the ratio of propaga- 
tion varies directly as the degree of sustentation above a certain minimum (and 
perhaps below 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 by natural selection. But among 
those that are equally adapted to the environment the ratio of propagation varies 
directly as the ratio of fertility. This propagation according to degrees of fertility 
is what I call the law of cumulative fertility. (See Jour. Linnean Society, Zodlogy, 
vol. XX, pp. 247, 248.) 
In my paper on Intensive Segregation, published in 18809, I discussed 
this principle under the term ‘‘ fecundal intension,”’ which I still retain 
to designate the influence of the principle in transforming races and 
species, which was the point of view chiefly considered in that paper. 
I there called attention to the fact that if in an isolated portion of a 
species the type of variation that attains the highest fertility is not 
