which changes or color
and design in butterflies may tend. Eimer finds the reason of this
small number of directions, in which development may proceed, in the
fact "that the elementary external influences of climate and
nourishment on the constitution of the organism are everywhere the
cause of the transformations."

Another important point is the difference of sex. If the butterflies
are of different sex, the males as a rule exhibit a more developed
stage of design and color than the females. These frequently present on
the upper side the stage of coloration, which the males present on the
lower side, while the upper side of the males is one stage in advance.
It is of special significance that the characters of the more advanced
sex frequently correspond to those of a related, superior species, and
occasionally to those of widely separated species. Eimer endeavors to
explain male predominance "by a more delicate and more developed, i.e.,
more complex, chemico-physical organization of the male organism." Even
this development tends toward simplification, the origin of dull-black
colors.

This most interesting question brings Eimer into conflict with another
Darwinian principle, the so-called principle of "sexual election,"
according to which the more striking characteristics of the male sex
become strengthened for the reason that females invariably give the
preference to the males endowed with them, over those that are less
"attractive." These exceedingly romantic ideas have been often and
deservedly repudiated, e.g., even by Wallace only a short time after
their first appearance. Eimer really does them too much honor when he
again undertakes, even with a certain amount of respect, a thorough
refutation of them, "as in every regard unfounded." It is of primary
importance to note here, that in the case of dimorphism of the sexes
abrupt modifications occur in connection with unilateral heredity. "It
is impossible for sexual selection to produce a change of design and
color, which results in the sudden kaleidoscopic formation of wholly
different designs, as we find actually taking place through the action
of artificial heat and cold and other factors in nature."

This brings us to a brief consideration of the answer, which Eimer
proposes to give to the question of the real causes of the formation of
species among butterflies. A precise and clear statement of this
important part of Eimer's theory of Descent, is contained