246 RESEARCH SEMINAR. 



July 29. The Mutation Theory of De Vries. By HERBERT J. 



WEBBER. 



The speaker called attention to the fact that very many of the 

 races and varieties of our cultivated plants have originated as 

 sudden variations. Individual variations were discussed in com- 

 parison with mutations and it was claimed that no strict line of 

 demarkation can be drawn between the two. The work of selec- 

 tions deals mainly with slight individual variations, some of which 

 as shown by experiment have strong transmitting power and re- 

 produce themselves true to type in large degree. No means 

 exists of distinguishing between these and small mutations which 

 are mainly transmitted true through the seed. The difference is 

 one of degree only, it was claimed, and cannot be detected with 

 certainity. 



The influence of natural selection in the origin of natural species 

 is not eliminated by accepting the idea that variations that form 

 species are produced as mutations. Mutations of all kinds are 

 doubtless formed, desirable and undesirable, fit and unfit, and 

 only those maintain themselves and form permanent species that 

 are fully in harmony with the environment and thus survive. 

 Undesirable mutations are weeded out by natural selection. Even 

 granting the occurrence of a mutation strikingly different from 

 the parent type and thoroughly fitted to the environment, some- 

 thing more is necessary other than natural selection to insure its 

 forming a new species. It is of primary importance that the 

 mutation or variation have strong transmitting power, giving 

 progeny like the parent mutation. Aside from this, some form 

 of isolation is necessary to secure the formation of the new type 

 as the few plants showing the variation would be swamped by pan- 

 mixia. This swamping the speaker pointed out could be over- 

 come or avoided in at least three ways ; namely, (i) geographical 

 isolation, (2) tendency to self-fertility, or prepotency of pollen, 

 (3) a tendency to prepotency or preponderance of type. The 

 first of these, the influence of geographical isolation, has been 

 emphasized by Gulich and Romanes, and its general application 

 is familiar to all through the classical illustration of snails in dif- 

 ferent valleys in the Sandwich Islands given by Gulich. The 

 influence of the other two factors the speaker has never seen 



