314 COOK 



active principle of evolution in natural selection or in mutation, 

 the kinetic theory finds evolutionary causes in normal diversity 

 and free interbreeding in specific networks of descent. 



Both the selection theory and the mutation theory imply that 

 new characters and new types have to be preserved by isolation. 

 Under the kinetic theory it is clearly perceived that isolation 

 explains only the multiplication of species, but is not an evolu- 

 tionary factor, or even a necessary condition of evolution. The 

 kinetic theory provides for the first time a consistent outline of 

 a method of gradual and continuous evolution in normally ex- 

 tensive, freely interbreeding specific groups, the condition in 

 which organisms everywhere exist in nature. 



PRINCIPAL AGENT OF EVOLUTIONARY CHANGE. 



At this point the various theories show, perhaps, their most 

 obvious divergencies. The doctrine of pure selection, or Dar- 

 winism, holds that selection is the actual cause or principle of 

 evolutionary advance, supporting this by various other assump- 

 tions, such as an environmental causation of variations or a cor- 

 relation between useful and useless variations. 1 



The isolation theory of Gulick appreciates the inadequacy of 

 selection and seeks for special conditions or behavior which can 

 explain the evolutionary progress of groups of individuals 

 which have merely been isolated from the parent species with- 

 out having been placed in appreciably different environments. 

 The Lamarckian doctrine of direct adaptation finds its greatest ad- 

 vantage here, in that the environment itself is supposed to cause 

 the changes directly. Professor De Vries argues, in some of his 

 writings, that mutations are due to environmental causes, though 

 frankly admitting that the connection of events is unknown. 



1 Belief in correlation of characters as an important adjunct to selective evo- 

 lution has been reaffirmed very recently by Professor Lank ester. 



"For they [correlated characters] enable us to understand how it is that 

 specific characters, those seen and noted on the surface by systematists, are not 

 adaptations of selective value. They also open a wide vista of incipient and 

 useless developments which may suddenly, in their turn, be seized upon by ever, 

 watchful natural selection and raised to a high pitch of growth and function." 

 See Lankester, E. Ray, 1906. Inaugural Address before the British Association 

 for the Advancement of Science. Science, N. S., 24: 228. 



