Aprit 19, 1912] 
single unit character would be sufficient to 
give a distinct and recognizable race or 
variety. Indeed, the difference between 
two varieties of a single unit character 
might mean that one variety would be 
exceedingly valuable and the other prac- 
tically worthless. De Vries asserts that 
unit characters are discontinuous in inherit- 
ance and do not exhibit transitional forms. 
A plant can not be hairy and at the same 
time smooth, or a fruit yellow and at the 
same time red. While there is yet much 
difference of opinion on these questions the 
preponderance of evidence certainly favors 
the unit character conception. 
If, then, we recognize that species are 
made up of unit characters and that dif- 
ferent species differ in the possession of 
different unit characters, the great problem 
in the evolution of species becomes the 
question of how the new unit character is 
acquired. Have all unit characters existed 
from the beginning, or are new unit char- 
acters beimg continuously acquired? A 
few years ago we supposed that new char- 
acters, if acquired in any form, must be 
seized upon, as it were, by natural selec- 
tion and preserved, or otherwise that they 
would beswamped by intercrossing and lost. 
We now know from Mendelian analysis that 
a unit character may be apparently lost in 
crossing, owing to the prevailing presence 
of its dominant allelomorph, but that in 
reality it is not lost or apparently changed 
and will reappear again when it happens 
that two gametes both bearing the character 
meet in fecundation. It may remain hid- 
den for many years, but as we are now 
inclined to view the matter, the character 
or the determiner of the character would not 
be permanently lost to the species unless all 
individuals possessing it were killed before 
they produced seed. This unit character 
idea would lead us to the conception of the 
species as made up of all the unit char- 
SCIENCE 
599 
acters that it has acquired by any means 
in its development and which still exist. 
The acquirement of any new unit character 
would add one more character to the spe- 
cies and double the number of possible 
varieties or races of the species. 
In evolutionary studies we have long 
recognized that variation was the founda- 
tion of evolution and that no evolution was 
possible without variation, but we have 
assigned to selection an all-important part 
as guiding and even stimulating the varia- 
tion in a certain direction. Darwin and 
particularly some of his more radical. fol- 
lowers have assigned to selection a creative 
force, in that it has been assumed that when 
nature by a slight variation gave the hint 
of a possible change in a certain direction, 
natural or artificial selection, by choosing 
this variant and selecting from among its 
progeny the most markedly similar vari- 
ants, could force the advance of the varia- 
tion in the direction indicated. Since 
Darwin’s time this cumulative action of 
selection has been emphasized so forcibly 
that we had come to recognize selection as 
an active force in creation rather than 
simply as a selective agency. To be the 
vital principle of evolution, as we now 
understand the species as made up of herit- 
able unit characters, the selectionist must 
show that a new character can be created 
by selection, otherwise selection becomes a 
secondary principle. 
When viewed from the standpoint of the 
production of a new and definitely herit- 
able unit which mendelizes, the task of 
selection becomes more doubtful. Dar- 
win’s idea, that changes in species required 
many years and probably many centuries 
for accomplishment, took the subject 
largely out of the field of experimentation 
and in a measure developed a speculative 
science. One of the greatest contributions 
to science made by De Vries was to estab- 
