s of Allen, Ridgway,
Gulick, and others, shows the value of isolation or segregation in
special areas as a factor in the origination of varieties and species,
the result being the prevention of interbreeding, which would otherwise
swamp the incipient varieties.

Here might be cited Delboeuf's law:[238]

"When a modification is produced in a very small number of
individuals, this modification, even were it advantageous, would be
destroyed by heredity, as the favored individuals would be obliged
to unite with the unmodified individuals. _Il n'en est rien,
cependant._ However great may be the number of forms similar to it,
and however small may be the number of dissimilar individuals which
would give rise to an isolated individual, we can always, while
admitting that the different generations are propagated under the
same conditions, meet with a number of generations at the end of
which the sum total of the modified individuals will surpass that of
the unmodified individuals." Giard adds that this law is capable of
mathematical demonstration. "Thus the continuity or even the
periodicity of action of a primary factor, such, for example, as a
variation of the _milieu_, shows us the necessary and sufficient
condition under which a variety or species originates without the
aid of any secondary factor."

Semper,[239] an eminent zooelogist and morphologist, who also was the
first (in 1863) to criticise Darwin's theory of the mode of formation of
coral atolls, though not referring to Lamarck, published a strong,
catholic, and original book, which is in general essentially Lamarckian,
while not undervaluing Darwin's principle of natural selection. "It
appears to me," he says, in the preface, "that of all the properties of
the animal organism, Variability is that which may first and most easily
be traced by exact investigation to its efficient causes."

"By a rearrangement of the materials of his argument, however, we
obtain, as I conceive, convincing proof that external conditions can
exert not only a very powerful selective force, but a transforming
one as well, although it must be the more limited of the two.

"An organ no longer needed for its original purpose may adapt itself
to the altered circumstances, and alter correspondingly if it
contains within itself, as I have explained above, the elements of
such a change. Then the influence exerted by the changed conditions
wi