444 



SCIENCE. 



[N. S. Vol. XVII. No. 429. 



experimental history of them is given in 

 the first volume of his brilliant work, 'Die 

 Mutationstheorie. ' These forms, he con- 

 tends, are true elementary species. That 

 is, they have new specific characters. These 

 characters are heritable. It does not mat- 

 ter whether these characters are large or 

 small— they become phylogenetic. These 

 plants having the liew specific characters 

 may not be species in the Linnaean or his- 

 toric or morphological sense, but they are 

 real entities. We must give up the his- 

 torical view of species when we study the 

 evolution of organic forms. Historic or 

 Lirmffian, species axe taxonomic concep- 

 tions ; the evolutional or elementary species 

 are physiological conceptions. 



The different categories of species, as 

 respects their origin, are given as follows 

 by De Vries : 



A. Origin by means of formation of new 



characters, or progressive species- 

 origin. 



B. Origin without formation of new 



characters. 



1. By the becoming latent {latentwer- 



den) of present characteristics, 

 or retrogressive species-origin. 

 Atavism in part belongs here. 



2. By the becoming active (fflcfwirMwgr) 



of latent characteristics, or de- 

 gressive species-origin. 



(a) Taxonomic anomalies. 



( h ) Real atavism. 



3. By means of hybrids. 



It will now be seen that the mutation 

 theory of De Vries, which is in some re- 

 spects a rephrasing and an extending of 

 the old idea of sports, does not of itself in- 

 troduce any new theory of the dynamics of 

 evolution. It is not a theory of heredity 

 nor of variation. His hypothesis of 'intra- 

 cellular pangenesis' carries the explana- 

 tion of these phenomena one step farther 

 back, however. The plant cells give off 

 pangenes. Bach of these pangenes divides 



into two. Ordinarily,, these two resemble 

 the parent; but now and then one of 

 them takes on a new character — the two 

 become unlike — and gives rise to a muta- 

 tion. This hypothesis, like Darwin's 

 pangenesis, is useful as a graphic basis 

 for discussion, whether or no it has real 

 physiological foundation. 



The most emphatic points of the muta- 

 tion theory, as they appeal to me, are these : 

 (1) It classifies variation into kinds that 

 are concerned in evolution and kinds that 

 are not ; and thereby it denies that all adap- 

 tation to environment makes for the pro- 

 gress of the race. (2) It denies the power 

 of natural selection to fix, to heap up or 

 to augment differences until they become 

 truly specific. (3) It separates the results 

 of struggle for existence and survival of 

 the fittest into two categories, only one of 

 which has an effect on phylogeny. (4) It 

 asserts that evolution takes place by steps, 

 small or great, and not by a gradual un- 

 folding or evolving of one form into an- 

 other. (5) It enforces the importance of 

 critical comparative study of great num- 

 bers of individual plants or animals. (6) 

 It challenges the validity of the customary 

 conception of species as competent to eluci- 

 date the method of evolution. 



There will arise confusion, in the forth- 

 coming discussions of the theory of dis- 

 continuity, as to what is a species; but 

 this confusion is not new. There are two 

 conceptions of species: (1) As taxonomic 

 groups, more or less arbitrarily made for 

 purposes of classification; (2) as real 

 things, marked by recordable differences, 

 however small or great, and conceived to be 

 the actual steps in the phylogeny- of the 

 race. These categories are so distinct that 

 they would not be confounded except for 

 the unfortimate circumstance that we use 

 one word (species) for the two. There has 

 been a growing conviction that the two 



