n one is overmastered by the
dogma of natural selection he is apt, perhaps unconsciously, to give
up all effort to work out the factors of evolution, or to seek to
work out this or that cause of variation. Trusting too implicitly to
the supposed _vera causa_, one may close his eyes to the effects of
change of environment or to the necessity of constant attempts to
discover the real cause of this or that variation, the reduction or
increase in size of this or that organ; or become insensible to the
value of experiments. Were the dogma of natural selection to become
universally accepted, further progress would cease, and biology
would tend to relapse into a stage of atrophy and degeneration. On
the other hand, a revival of Lamarckism in its modern form, and a
critical and doubting attitude towards natural selection as an
efficient cause, will keep alive discussion and investigation, and
especially, if resort be had to experimentation, will carry up to a
higher plane the status of philosophical biology."

Although now the leader of the Neodarwinians, and fully assured of the
"all-sufficiency" of natural selection, the veteran biologist Weismann,
whose earlier works were such epoch-making contributions to insect
embryology, was, when active as an investigator, a strong advocate of
the Lamarckian factors. In his masterly work, _Studies in the Theory of
Descent_[227] (1875), although accepting Darwin's principle of natural
selection, he also relied on "the transforming influence of direct
action as upheld by Lamarck," although he adds, "its extent cannot as
yet be estimated with any certainty." He concluded from his studies in
seasonal dimorphism, "that differences of specific value can originate
through the direct action of external conditions of life only." While
conceding that sexual selection plays a very important part in the
markings and coloring of butterflies, he adds "that a change produced
directly by climate may be still further increased by sexual selection."
He also inquired into the origin of variability, and held that it can be
elucidated by seasonal dimorphism. He thus formulated the chief results
of his investigations: "A species is only caused to change through the
influence of changing external conditions of life, this change being in
a fixed direction which entirely depends on the physical nature of the
varying organism, and is different in different species or even in the
two