ON THE QUICK GROWING WALNUT 
endeavoring to gain a clear notion as to just what 
are the underlying principles that determine 
whether or not a certain heritable character or pair 
of characters shall Mendelize; and in so doing we 
may correlate our earlier studies, and secure a 
clearer notion of the underlying principles of 
evolution, and of the origin and development of 
species, than could perhaps have been gained with- 
out the aid of the illustrative cases that have been 
presented. 
NATURAL SELECTION 
In the preceding chapter we briefly reviewed 
the story of the vicissitudes to which plant life has 
been subjected in the course of recent geological 
eras. We were concerned there with the elimina- 
tion of unadaptable species rather than with the 
evolution of adaptable ones. 
But it should of course be understood that the 
same principle of natural selection applies to the 
preservation and to the weeding out of species. 
In the case under consideration, it was the 
changed climatic conditions, through which the 
northern hemisphere was transformed from a re- 
gion of tropical heat to one of arctic cold, that 
resulted in the destruction of countless species, 
leaving only a tithe of the original number to con- 
stitute the flora of the temperate zone in our own 
day. 
[203] 
