smann,
neither De Vries nor Roux, has done this. Naegeli, in his
_Mechanisch-Physiologische Theorie der Abstammungslehre_[130] which is
to a great extent in agreement with Weismann, constructed a theory of
the idioplasm, that represents it (like the germ-plasm) as developing
continuously in a definite direction from internal causes. But his
internal "principle of progress" is at the bottom just as teleological
as the vital force of the Vitalists, and the micella structure of the
idioplasm is just as hypothetical as the "dominant" structure of the
germ-plasm. In 1889 Moritz Wagner sought to explain the origin of
species by migration and isolation, and on that basis constructed a
special "migration-theory." This, however, is not out of harmony with
the theory of selection. It merely elevates one single factor in the
theory to a predominant position. Isolation is only a special case of
selection, as I had pointed out in the fifteenth chapter of my
_Natural history of creation_. The "mutation-theory" of De Vries,[131]
that would explain the origin of species by sudden and saltatory
variations rather than by gradual modification, is regarded by many
botanists as a great step in advance, but it is generally rejected by
zoologists. It affords no explanation of the facts of adaptation, and
has no causal value.

Much more important than these theories is that of Wilhelm Roux[132]
of "the struggle of parts within the organism, a supplementation of
the theory of mechanical adaptation." He explains the functional
autoformation of the purposive structure by a combination of Darwin's
principle of selection with Lamarck's idea of transformative heredity,
and applies the two in conjunction to the facts of histology. He lays
stress on the significance of functional adaptation, which I had
described in 1866, under the head of cumulative adaptation, as the
most important factor in evolution. Pointing out its influence in the
cell-life of the tissues, he puts "cellular selection" above "personal
selection," and shows how the finest conceivable adaptations in the
structure of the tissue may be brought about quite mechanically,
without preconceived plan. This "mechanical teleology" is a valuable
extension of Darwin's monistic principle of selection to the whole
field of cellular physiology and histology, and is wholly destructive
of dualistic vitalism.

The most important advance that evolution has made since Darwin and
the most valuable