Summary and General Conclusions 455 
from internal causes is so regulated that only adaptive struc- 
tures arise (although only adaptive ones may survive). 
Our general conclusion is then as follows: A species does not 
arise from another one because it is better adapted. Selection, 
in other words, does not account for the origin of new species ; 
and adaptation cannot be taken as the measure of a species. 
It may sound like a commonplace to state that only those 
individuals survive and propagate themselves that can find 
some place in nature where they can exist and leave descen- 
dants; and yet this statement may contain all that it is 
necessary to assume, in order to account for the fact that 
organisms are, on the whole, adapted. Let us see how this 
view differs from the Darwinian statement of the origination 
of new forms through a process of natural selection. 
According to Darwin’s view of the origin of species, each 
new species is gradually formed out of an older one, because 
of the advantage that the new individual may have over the 
parent form. Each step forward is acquired, because it 
better adapts the organism to the old, or to a new set of 
conditions. In contrast to this, I have urged that the for- 
mation of the new species is, as a rule, quite independent 
of its adaptive value in regard to the parent species. But 
after it has appeared, its survival will depend upon whether 
it can find a place in nature where it can exist and leave 
descendants. If it should be well adapted to an environment, 
it will be represented in it by a large number of individuals. 
If it is poorly adapted, it may only barely succeed in existing, 
and leave correspondingly fewer descendants. If its adap- 
tiveness falls below a certain point, it can never get a perma- 
nent foothold, however often it may appear. Thus the test 
of survival determines which species can remain in existence, 
and which cannot, but new species are not manufactured in 
this way. How far subsequent variations may be supposed 
to be determined by the survival of certain species and the 
destruction of others will be discussed presently. 
