REVIEWS. 
121 
In his account of the origin of species Darwin took varieties for 
granted ; he gave little or no account of the process of variation, but 
showed how by the survival of the fittest species may arise from 
varieties, no matter how slightly the latter differ from their congeners 
at any particular stage. 
It was not until some time after the appearance of this theory that 
the distinction between continuous and discontinuous variation came 
to be clearly recognized. Wallace was the first to give it as his 
opinion that species arise only by the continuous method of develop¬ 
ment, refusing any assistance from sudden variations. And this view 
has remained for many years the prevailing one. Recently, however, 
several voices have been uplifted against it, and with increasing 
degrees of loudness. In 1889 Galton wrote as follows : “ The theory 
of natural selection might dispense with a restriction for which it 
is difficult to see either the need or the justification, namely, that the 
course of evolution always proceeds by steps that are severally 
minute, and that become effective only through accumulation.” And 
a few years later Bateson, in his “ Materials for the Study of 
Variation,” summed up his own conclusion in the following words : 
“ The evidence of variation suggests, in brief, that the discontinuity 
of species results from the discontinuity of variation.” In the present 
work De Vries goes still further, for he brings forward weighty 
arguments to prove that the selection of continuous variations does 
not lead to a production of specific differences. It would no doubt 
be rash to hastily accept such a conclusion ; nevertheless it must 
be admitted that a great deal of excellent evidence is adduced by 
De Yries in support of his opinion. 
The problem of specific differences may be attacked by three 
distinct methods. The first or comparative method was exclusively 
used by the older systematists. Galton introduced the use of statistics 
in biological problems to English science. The third method, that 
of direct experiment , was effectively employed by hybridizers even 
before the time of Darwin ; but in recent years it had largely 
been discarded. De Vries makes valuable use of all three methods, 
but he rightly lays greatest stress upon the last. 
“ Die Mutations-theorie ” begins with a critical examination of 
previous views, in the course of which the introductory questions 
already alluded to are discussed at considerable length. An important 
distinction is drawn between the elemental species of Jordan and the 
aggregate species of Linnæus. Linnean species are the somewhat 
comprehensive groups of plants which have been long accepted by 
botanists as the units of classification. Such species frequently in¬ 
clude a number of minor forms, which differ from one another 
distinctly, though only to a relatively small extent, and which come 
true to their type when grown from seed. Jordan described more than 
two hundred such forms within the limits of the single Linnean species, 
8(1)4 (16) 
