498 MEMOIRS OF THE CARNEGIE MUSEUM 
1. The Mutation Theory of De Vries. 
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 the 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 Vries 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 Monthly, vol. 39, 
1906, p. 207). And since De Vries believes that species are formed only by muta- 
tion, it should. be expected that the morphological differences between existing 
species should at least frequently exhibit signs of ‘sudden leaps.” If such leaps 
are observed in our species of Cambarus, this would tend to support this part of De 
Vries’ theory ; if not, the theory that mutations are always or generally marked by 
discontinuity of variation, should be dropped. 
2. Species, Varieties, and Variations among the Pennsylvania Crawfishes. 
I have distinguished in the systematic part of this 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 the reasons for thus estimating them should 
be given. . 
The seven species of Pennsylvania belong to two subgenera, Faxonius and 
Bartomus, which are distinguished by very sharp differences in the male copulatory 
organs. 
