EFFECT OF NATURAL SELECTION 93 
not only to develop still further their naturally valuable qualities, 
but to bring about more or less radical readjustments occasioned 
and made necessary by these new demands of ours. 
Natural selection always at work. We must not for a mo- 
ment suppose that our domestication and the new standards of 
breeding entirely do away with natural selection. In respect to 
tooth and claw, of course selection stops the moment we make 
warfare impossible, but in such 
fundamental matters as constitu- 
tional vigor, fecundity, and the 
vital and reproductive faculties 
natural selection never surren- 
ders its hold upon a species. 
Ofttimes we forget this and 
are brought up standing by the 
consequences. Sometimes our 
standards of selection are unwit- 
tingly at opposites with these 
fundamental matters, and then 
the shock and the lesson are 
severe. For instance, many an 
amateur breeder will select the 
fattest and smoothest pigs for Fic. 13. The passenger pigeon, 
breeding purposes, not knowing SIC Perot Shen dorcloped by 
that these are neither the most selection (see Figs. 14 and 15) 
prolific nor the hardiest. His 
herd soon runs out. Natural selection has been at work day 
and night to undermine his herd at the point of infertility. 
Some very favorite strains of cattle or sheep are decidedly 
"shy breeders.” If so, it may as well be understood that they 
will go down under the relentless work of natural selection, 
unless indeed the defect can be speedily remedied by finding 
prolific strains among the favorites. 
Power of selection to modify type. Selection can do far 
more than develop a single type to conform to some single 
