eas, is perhaps best seen in the fact that not one of his many
successors has succeeded in modifying his theory of descent in any
essential point or in discovering an entirely new standpoint in the
interpretation of the organic world. Neither Nageli nor Weismann,
neither De Vries nor Roux, has done this. Nageli, in his
"Mechanisch-Physiologische Theorie der Abstammungslehre" (Munich,
1884.), 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 micellar 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 ("Die Mutationstheorie", Leipzig, 1903.), 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 ("Der
Kampf der Theile im Organismus", Leipzig, 1881.) 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
b