PEOFESSOR DE VEIES ON THE ORIGIN OF 
SPECIES\ 
By W. F. R. WELDON, F.R.S. 
In the first volume of his Mutationstheorie Professor de Vries has defined and 
ilhistrated his conception of the fundamental phenomena on which the process of 
organic evolution depends. He has done this so fully that it seems permissible to 
discuss the essential features of his position without waiting for his promised 
second volume. 
Professor de Vries takes the refreshingly unusual course of trying to make 
clear at the outset what he means by a species. The ultimate systematic unit 
which he recognises is the " elementary species," or the limited species of such 
botanists as Jordan, such zoologists as Bourguignat — the smallest group of indivi- 
duals which can be shown to differ, and to produce offspring which differ, from 
other groups in any certain number of characters. The characters of such elemen- 
tary species are normally constant through successive generations. It is usually 
possible to arrange a number of such elementary species in a series, so that each 
species, although it differs from its neighbours in each of a, generally lai'ge, 
number of characters, does so to a very slight extent, and the series is therefore 
nearly continuous. Such a series of groups forms a Linnean species, expressed by 
the ordinary binomial nomenclature. In some cases the boundaries of a Linnean 
species are mere matters of cpnvention (" Sache des sogenannten systematischen 
Tactes "), in others there are at intervals gaps in the series of elementary species 
which form natural boundaries for the Linnean groups. Such gaps are in general 
due to the extinction of previously existing elementary species. " Die Linne-schen 
Arten entstehen durch den Untergang einzelner elementaren Arten aus der bis 
dahin ununterbrochener Reihe. Dieses Entstehen ist also ein rein historischer 
Vorgang, und kann nie die Gegenstand experimenteller Forschung werden." 
(p. 44.) 
* Die Mutationstheorie, Versuche und Beohachtungen iiber die Entstehung der Arten im Pjlanzen- 
reich, Bd. 1. Leipzig, 1901. 
