UNBALANCED PROPAGATION. 149 



brought about either by the unbalanced effects of the processes sepa- 

 rating the individuals into coexistent groups that are prevented from 

 intergenerating, or by the unbalanced effects of differing degrees of 

 survival for different forms of variation. The former principle is called 

 "isolation," and the latter principle "selection." It is quite evident 

 that in as far as selection prevents any form from propagating, in so 

 far it prevents intergeneration between that form and the forms that 

 produce the next generation ; but, at the same time, I prefer to define 

 isolation as the prevention of free crossing between coexisting groups, 

 though the individuals of each group, so far as they survive, are 

 freely intergenerating. When pointing out the correspondences 

 between selection and isolation, I would say that both are principles 

 by which the abiding principle of segregate breeding is modified 

 and intensified; and that when either of them produces unbalanced 

 propagation effected by the same sign in successive generations, the 

 result is transformation of type. 



During the process of domestication the reproductive powers of 

 many species are so impaired that it is with difficulty that a perma- 

 nent domestic race can be produced. Many individuals that thrive 

 on the nourishment furnished fail to leave offspring, so that the race 

 is perpetuated not by the offspring of those which are most pleasing to 

 those who keep and select them, but by the offspring of those which 

 have offspring. The same principle may produce transformation in 

 species that are not under domestication. For if, among the many 

 varieties, there arises one that, while retaining equal adaptation, is 

 more fruitful than other varieties, it will be favored by fecundal selec- 

 tion. The descendants of the most fertile will have the largest share 

 in producing the next generation. This will tend to produce increas- 

 ing fecundity in succeeding generations. This is a form of discriminate 

 survival ; but we must remember that this fecundal selection will pro- 

 duce accumulation of other characters besides fecundity only when 

 fecundity is correlated with certain variations that do not represent 

 the typical or average form ; that is, only when it is unbalanced fecun- 

 dity. This seems to be a necessary law. As a corollary from this law, 

 I judge that, in a stable intergenerating species or variety, the aver- 

 age form will be found to be most fertile ; or, at least, the forms that 

 depart from the average will not be continuously endowed with higher 

 fertility than the average form. In considering the effect of selective 

 survival we have to discriminate between balanced and unbalanced 

 selection. Unbalanced selection is either the selection of individuals 

 above the average producing an increase of the character thus selected 

 or the selection of individuals below the average producing a decrease 



