174 THE PROBLEMS OF EVOLUTION 



nary relationships. It is no less probable that quan- 

 titative changes in an organism may lead ultimately 

 to conditions in the organism which enable it to 

 take advantage of additional possibilities of the 

 environment, and thus to truly qualitative change. 



We are also forced to consider whether changes 

 of any type can be indefinitely cumulative. The 

 best answer from the point of view of evolution is 

 that cumulative change is very common in the long 

 phylogenetic series of fossil remains and that it is 

 evident in individual responses. 



There is a degree of speculation in all of these 

 points, but they serve the valuable purpose of in- 

 dicating how gradual is the transition between 

 apparently different concepts. When we consider 

 them, it is less easy to trust the common idea of 

 mutations. Even new characters take on a mean- 

 ing entirely different from the direct implication 

 of the word new, and the imperceptible gradation 

 which must be a prominent feature of evolutionary 

 processes stands out in its true perspective. 



Through this as through other approaches we 

 arrive ultimately at the same point. No existing 

 interpretation of evolutionary processes is an ade- 

 quate explanation. Some are wholly satisfactory 

 as partial interpretations of things which have 

 been observed in nature and in the laboratory, 

 but even the association of these valid partial ex- 

 planations does not provide a sound and satisfying 



