498 MEMOIRS OF THE CARNEGIE MUSEUM 



1. TIte Mntation Theorij of De Vrics. 



The latest fashion in evolution theories is the so-called " mutation theory "' of 

 De Vries (De Vries, 1905). It is much discussed at present, and the general trend 

 of opinion is that, although De Vries' idea of tlie origin of species may not hold 

 good in all cases, he certainly has demonstrated at least one way by which species 

 may be formed. It is generally maintained with emphasis that his experiments 

 are beyond doubt and that the facts demonstrated by him cannot be denied. 



This indeed is the case, and it would be lamentable if any of the statements pre- 

 sented by De Vries as facts should prove to be unreliable. I am decidedly of the 

 opinion that the statements are correct, but I also hold that De Vries was not the 

 first to bring the facts forward. They belong to a class that was known long ago. 

 But furthermore, I believe that the conclusions drawn by De Vries from these 

 facts are entirely wrong. 



I recently have devoted several articles to demonstrate this, and shall not again 

 go into detail here {see Science, May 11, August 17, and November 30, 1906). 



However, I shall discuss here a special part of De Vries' theory, which concerns 

 the distinction he makes between " fluctuating variation " and " mutation." The 

 latter is said to be characterized by "sudden leaps,'' while the former is said to be 

 by " small steps." Although De \'ries sometimes does not lay much stress upon 

 this distinction (see Copeland, 1904, p. 421), this difference is often regarded as 

 paramount in his theory (see MacDougal, in Popular Science Monildjj, vol. 39, 

 1906, p. 207). And since De Vries believes that species are formed only by muta- 

 tion, it sliould be expected that the morphological differences betw^een existing 

 species should at least frequently exhibit signs of "sudden leaps." If such leaps 

 are observed in our species of Caviharus, this would tend to support this part of De 

 Vries' theory ; if not, the theory that mutations are alwaj's or generall}' marked l)y 

 discontinuity of variation, should be dropped. 



2. Species, Varieties, and Variations among the Pennsjilvavia Crawfishes. 



I have distinguished in the systematic part of tliis monograph seven species and 

 one variety among the Pennsylvania crawfishes. Besides I have discussed another 

 extralimital variety. This means that the characters distinguishing these forms are 

 different in their taxonomic value, and tlie reasons for thus estimating them should 

 be given. 



The seven species of Pennsylvania belong to two subgenera, Faxonius and 

 Bartonius, which are distinguished by very sharp differences in the male copulatory 



