EVOLUTION 445 



merely observations made while collecting seed for quite 

 different purposes, but they serve to illustrate what I 

 think to be a very important law, namely : Fertility is not 

 uniformly distributed among all individuals, but for stable 

 races tJiere is a strong tendency for the character of maximum 

 fertility to become one zuith the character zuhich is the type. 

 Thus any race as we find it is very largely the product 

 of its modal members, and not proportionately of all its 

 individual members ; its variability is not the potential 

 variability of the race, but deviates from this maximum 

 limit towards the minimum variability, i.e. that of the 

 progeny of a modal mating. 



Further, this condition of things explains with much 

 plausibility, why if any type of life be moved into a new 

 environment, there appears in the course of a few genera- 

 tions much increase of variability, for the old modal centre 

 of fertility will alter with the new environment and non- 

 modal, even extreme values of a character may now 

 become effectively fertile ; thus the racial as distinct from 

 the modal variability may for a time exhibit itself 



Now the problem which is thrust upon us, if the 

 above law be substantiated, is the following one : A 

 given environment connotes a certain type for a given 

 form of life, a type which with repeated selection 

 approaches (p. 405) the value of the character fittest to 

 the environment. This environment also causes 'some 

 value of the character to be of maximum fertility. Why 

 should the modal or type character and the most fertile 

 character be identical ? If fertility be simply proportional 

 to duration of life, then the identification of the modal 

 and most fertile characters becomes an obvious truism, 

 the individuals best fitted to survive will live to have 

 most offspring.^ But in many cases the individual only 



some plant, and then again the grades of fertiUty associated with this char- 

 acter, as by counting, weighing, or otherwise measuring the seed from the 

 same number of plants of each value of the character, or from the same number 

 of capsules with the same value of any character. 



1 I hope shortly to have definite quantitative measures of the correlation 

 between duration of life and fertility in the case of man. Mr. G. U. Yule 

 points out to me that there is probably more stress to be laid on this correla- 

 tion than I have indicated above. 



