92 PRESIDENTIAL ADDRESS SECTION' D. 



continuous as could be detected." Similar conclusions were>- 

 reached by R. W. Hegner. 



Castle wisely remarks that mutation and pure lines have a 

 limited applicability in the broad field of organic evolution. He 

 thinks that through selection or after selection a Mendelian unit 

 character can be changed. Selection may not be able to start new 

 lines of variation, yet it can continue and extend variation already 

 begun. Warren's work, already noted, has the same conclusion. 

 We thus see that, although during the last few years the import- 

 ance of the role of selection in evolution has declined in favour 

 and has even been doubted, largely due to De Vries' work on 

 mutations and to Johannsen's work on pure lines, yet according 

 to Jennings and to Castle the discontinuous variations ultimately 

 become continuous and selection is a process in evolution after all. 



Jennings sums up the matter thus: "Evolution, according to 

 the typical Darwinian scheme, through the occurrence of many 

 small variations and their guidance by natural selection, is per- 

 fectly consistent with what experimental and palaeontological. 

 studies show us; to me it appears more consistent with the data 

 than does any other theory." 



In the last sentence the evidence from palaeontological studies 

 was mentioned. Following Osborn, as interpreted by Jennings, iti 

 may be stated that in palaeontology the evidence is for evolution 

 by minute, continuous variations, which follow a definite trend or 

 course. There are, however, other variations from the line of 

 definite trend. 



Another palaeontologist, Dr. Bather, in his address on 

 "Fossils and Life" before the British Association last year, states 

 that in organisms the "changes of form are a reaction to the 

 stimuli of the outer world." He favours the view that "the life 

 history of races is a response to their environment." The "en- 

 vironment changes slowly and the response of the organism always 

 lags behind it." A living organism cannot be conceived apart 

 from its environment. Palaeontologists nowadays are the chief 

 opponents of the hypothesis of discontinuity. 



Osborn expresses the Lamarckian explanation of the pro- 

 cesses of evolution in modern terms thus: "The causes of genesis 

 of new form and new function are to be sought in the body cells." 

 He expresses the Darwinian explanation in the following terms: 

 "The genesis of new form and function is to be sought in the germ 

 cells or chromatin." These interpretations are probably much 

 more sharply distinct or antithetical than Lamarck or Darwin 

 would have made them, or even than the problem, being one of 

 living organisms, warrants. Now that we know of hormones, or 

 internal secretions, it is possible that such secretions of certain 

 parts of the soma, when affected by external stimulation, may 

 affect the germ cells. It may be stated then that selection may 

 take place through the action of external conditions. Indeed, 

 MacBride writes that "selection alone, when the environment 

 remains constant, is powerless to effect evolution." 



