THE MUTATION THEORY 



IOI 



change become. If the differences between species have not 

 come about by the addition or loss of factors one at a time, then 

 we must suppose that the changes have been effected by even 

 larger steps, and variations including groups of characters, must 

 be invoked. 



That changes of this latter order are really those by which 

 species arise, is the view with which de Vries has now made us 

 familiar by his writings on the Mutation Theory. In so far as 

 mutations may consist in meristic changes of many kinds and 

 in the loss of factors/it is unnecessary to repeat that we have 

 abundant evidence eft their frequent occurrence. That they may 

 also more rarely occur by the addition of a factor we are, I think, 

 compelled to believe, though as yet the evidence is almost en- 

 tirely circumstantial rather than direct. /The evidence for the 

 occurrence of those mutations of higher order, by which new 

 species characterized by several distinct features are created, 

 is far less strong, and after the best study of the records which 

 I have been able to make, I find myself unconvinced. The facts 

 alleged appear capable of other interpretations. 



The most famous and best studied examples are of course the 

 forms of Oenothera raised by de Vries from Oenothera Lamarckiana 

 in circumstances well known to all readers of genetic literature. 

 Whatever be the true significance of these extraordinary "mu- 

 tations" there can be no question about the great interest which 

 attaches to them, and the historical importance which they will 

 long preserve. Apart also from these considerations it is be- 

 coming more and more evident that in their peculiarities they 

 provide illustrations of physiological phenomena of the highest 

 consequence in the study of genetics at large. 



De Vries found, as is well known, that Oenothera Lamarckiana 

 gives off plants unlike itself. These mutational forms are of 

 several distinct and recognizable types which recur, and several 

 of them breed true from their first appearance. The obvious 

 difficulty, which in my judgment should make us unwilling at 

 present to accept these occurrences as proof of the genesis of new 

 species by mutation, is that we have as yet no certainty that the 

 appearance of the new forms is not an effect of the recombination 



