442 EVOLUTION AND GENETICS 



other steps in evolution. Modern thought on this increase of 

 capacity has crystaUized under the name of emergent evolution. 

 The name is scarcely necessary for the belief is no more than a 

 logical statement of things long held true. The role of sentient 

 powers and inherent directive forces has also gained a prominent 

 place in evolutionary thought, but here we are dangerously near 

 to passing out of the transitional zone into purely philosophical 

 speculation. 



Although discouraging it is not surprising that the method of 

 evolution should be so elusive, for it is the most complex and 

 baffling field of research. The genes with which the problem is 

 so intimately concerned are exceedingly small. They have never 

 been seen. Even the chromosomes must be subjected to extensive 

 treatment before they can be examined. Physicists may do as 

 they will with the atom and electron; these units are tractable in 

 comparison with genes, for methods can be devised for studying 

 them without destroj'ing them. Living matter presents more 

 difficulties. W hen chromosomes are made visible they are dead 

 and the gene remains a hypothetical unit in their densely stained 

 suljstance. 



In spite of these limitations the fact of evolution remains an 

 established principle of biology through all of the investigations 

 and disputes concerning its methods, and, knowing this to be so, 

 we may feel confident that we shall some day solve the problems 

 which are before us today. 



Summary. Considering all of the proposed theoretical explana- 

 tions of evolutionary method we find an almost universal lack of 

 explanation of the origin of variations. Darwin's theory of nat- 

 ural selection and the mutation theory together serve to explain 

 adaptation but must l^egin with variation. Lamarck's theory 

 offers the idea of interaction of heritage and environment but 

 fails to account for the extension of its results beyond the individ- 

 ual. An adequate theory of evolution must do both things, i.e., 

 it must account for changes and explain how they become the 

 characters of species. A logical analysis of the available facts 

 shows that the organism and all significant organic characters 

 are a product of the heritage reacting to the complex environ- 

 ment, and variations in the latter seem sufficient to account for 

 variations in the resulting living organisms. This view is essen- 

 tially the tetrakinetic theory of Osborn. We are able to see varia- 



