PAPERS PRESENTED AT GENERAL SESSIONS 35 



As an illustration of this, reference may be made to 

 the results of this work as bearing on the mutation ex- 

 planation. Until genetics began to uncover the machin- 

 ery of inheritance, which of course is fundamental in 

 producing variations, the general belief in evolution 

 included the following ideas : Inheritance of acquired 

 characters is exploded; Darwinian variations are du- 

 bious as a basis for explaining evolution; but mutation, 

 with natural selection among the mutants, doubtless ac- 

 counts for the facts. Now what does genetics tell us? 

 The majority of mutants may be called degenerates, the 

 new characteristics shown serving to adapt the mutant 

 more poorly to the environment than the parent was 

 adapted. In fact, the general statement is that the ma- 

 jority of mutants are much worse than their parents, 

 and none of them are better. If only a few were better 

 equipped, they would furnish sufficient material for evo- 

 lution; but with none better equipped, evolution is 

 blocked. 



Such considerations have made many biologists feel 

 less certain in explaining evolution than they were a few 

 years ago. This loss of faith in mutation, added to re- 

 cent discoveries on inheritance of acquired characters, 

 has caused many to seek an explanation of progressive 

 evolution in Lamarckian terms. 



The great problem we are facing is progressive evolu- 

 tion, commonly called "orthogenesis", which history 

 has made so evident. Continuous variations, discontin- 

 uous variations (so-called "mutations"), and hybrid 

 variations may all be explained as due to a complex of 

 factors. Such variations, however, are like the waves 

 on the surface of a choppy sea, running in every direc- 

 tion, and getting nowhere. Progressive evolution, how- 

 ever, may be likened to a deep-seated oceanic current 

 which moves steadily in one direction without any refer- 

 ence to the choppy surface. How can we explain this 

 oceanic current ? In my own field, I have been impressed 

 by the progressive evolution of the gymnospenns, of 

 which we have continuous records from the Paleozoic to 

 the present time. Throughout that tremendous stretch 

 of time, in spite of all imaginable changes in external 



