Variation and Heredity 291 
fittest species.” It is, therefore, not the study of Linnzan 
species that has a physiological interest, but it is the study 
of the elementary species of which the Linnzan species are 
made up, that furnishes the all-important problem for experi- 
mental study. 
De Vries gives a critical analysis of a number. of cases in 
which new races have been formed under domestication. He 
shows very convincingly that, whenever the result has been 
the outcome of the selection of fluctuating variations, the 
product that is formed can only be kept to its highest point 
of development by the most rigid and ever watchful care. 
If selection ceases for only a few generations, the new form 
sinks back at once to its original level. Many of our cul- 
tivated plants have really arisen, not by selection of this 
sort, but by mutations; and there are a number of recorded 
cases where the first and sudden appearance of a new form 
has been observed. In such cases as these there is no need 
for selection, for if left to themselves there is no return to the 
original form. If, however, after a new mutation has ap- 
peared in this way, we subject its fluctuating variations to 
selection, we can keep the new form up to its most extreme 
limit, but can do nothing more. 
Another means, frequently employed, by which new varie- 
ties have been formed is by bringing together different ele- 
mentary species under cultivation. For instance, there are a 
large number of wild elementary species of apples, and De 
Vries believes that our different races of apples owe their 
origin in part to these different wild forms. Crossing, culti- 
vation, and selection have done the rest. 
De Vries points out some of the inconsistencies of those 
who have attempted to discriminate between varieties and 
species. The only rule that can be adhered to is that a 
variety differs from a species to which it belongs in only one 
or in a few characters. Most so-called varieties in nature are 
really elementary species, which differ from their nearest 
