290 Evolution and Adaptation 
different elementary species of Drada verna are less different 
from each other than the forms of leaves on a tree. The 
essential differences between the two kinds of variation is 
that the mutation is constant, while the continuous variation 
fluctuates back and forth. 
The following example is given by De Vries to illustrate the 
general point of view in regard to varieties and species. The 
species Oxalis corniculata is a “collective” species that lives 
in New Zealand. It has been described as having seven well- 
characterized varieties which do not live together or have in- 
termediate forms. If we knew only this group, there would 
be no question that there are seven good species. But in 
other countries intermediate forms exist, which exactly bridge 
over the differences between the seven New Zealand forms. 
For this reason all the forms have been united in a single 
species. 
Another example is that of the fern, Lomaria procera, from 
New Zealand, Australia, South Africa, and South America. 
If the forms from only one country be considered, they appear 
to be different species; but if all the forms from the different 
parts of the world be taken into account, they constitute a 
connected group, and are united into one large species. 
It will be seen, therefore, that the limits of a collective 
species are determined solely by the deficiencies in the ge- 
nealogical tree of the elementary species. If all the element- 
ary species in one country were destroyed, then the forms 
living in other countries that had been previously held 
together because of those which have now been destroyed, 
would, after the destruction, become true species. In other 
words: “The Linnean species are formed by the disap- 
pearance of other elementary species, which at first connected 
all forms. This mode of origin is a purely historical process, 
and can never become the subject of experimental investi- 
gation.” Spencer’s famous expression, the “survival of the 
fittest,” is incomplete, and should read the “survival of the 
