ON THE QUICK GROWING WALNUT 
All this has to do, however, with the destruc- 
tion, rather than with the evolution of species. 
I have already said that the principles of nat- 
ural selection apply with equal force, and seem- 
ingly with entire impartiality, to the destruction 
and to the preservation of species. 
But it is obvious that mere preservation of 
species does not necessarily imply also the evolu- 
tion of species. Natural selection might give a 
dominant position to a particular species, and pre- 
serve it for indefinite periods without essential 
change. 
But this could only occur in case the condi- 
tions of environment themselves remained essen- 
tially unchanged. 
It is fundamental to a clear understanding of 
evolution to realize that in a changing environ- 
ment, under natural conditions, no species could 
be preserved unless it proved adaptable. 
Indeed, the more perfectly adjusted the species 
might be to its environment at a given period, the 
more certainly must that species be destroyed 
should the essential conditions of the environment 
change. 
The great penalty of specialization is the 
danger that attends it from this source. It is held 
that the species that were eliminated when the 
great climatic change occurred to which we have 
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