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. 



