The Theory of Evolution 225 
Darwin was obliged in the Jater editions of the “Origin of 
Species” to assume that a beneficial variation, in order to get a 
foothold, must appear in a relatively large number of individuals. 
His critics were not-slow in pointing out that if we suppose a 
variation to be so common that it occurred in many individuals 
at the same time, it is probably the result of some definite process 
taking place, and the change into a new species might be brought 
about in time without selection of any kind being necessary. 
Hence one assumption will do instead of two, and the explanation 
is correspondingly simplified. If we still try to save Darwin’s 
theory by assuming that we need only suppose that half of the 
fluctuations in a given direction, z.e. those on one side of the 
mean, survive and furnish the basis for the next generation, and 
that this process repeats itself in every generation, we meet with 
a fact, apparently well established, that shows how futile it is to 
attempt to save the theory in this way. It has been shown by 
actual experiment that all that such a process accomplishes is 
to raise the average in the direction of selection, and that how- 
ever long the process is continued, and however severe the selec- 
tion, nothing new, no new species, can be created in this way. 
As soon as the selection ceases, the form quickly slips back to its 
original condition. 
Let us see if the mutation theory can surmount this difficulty, 
for if mutations appear in the small numbers shown by de Vries’s 
experiment, it may seem that they, too, might quickly become 
swamped by back-crossing with the parent type. 
In the first place the new type is at the start different from the 
parent, and may be capable of living in a different locality, 7.e. 
under somewhat different external conditions. Hence the chance 
of becoming swamped by back-crossing is lessened. If more 
than one individual occurs in the proper locality, the chance for 
propagation is given. Since the mutations in the evening prim- 
rose are produced year after year, this condition may at some 
time be realized. In hermaphroditic forms, moreover, the oppor- 
tunity of self-fertilization exists, and this increases the chance of 
the mutation establishing itself. 
Q 
