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 signin 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 isa 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. Asa 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 averageform. 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 
