ts of the hand or
foot come to full development. But whereas in forms with a normally
developed hand, _e.g._ the tortoise and man, all the digits develop and
differentiate at about the same rate, in forms which have in the adult
reduced digits, _e.g._ the ostrich and the pig, these vestigial digits
undergo a very slow and incomplete differentiation, while the others
develop rapidly and completely. He draws a general distinction between
organs that are phylogenetically progressive and such as are
phylogenetically regressive, and seeks to prove that progressive organs
show an ontogenetic acceleration and regressive organs a retardation.[535]
The acceleration or retardation affects not only the mass-growth of the
organs, but also their histological differentiation.

Now between progression and functioning and between regression and
functional atrophy there is obviously a close connection. Loss of
function is well known to be one of the chief causes of the degeneration
of organs in the individual life, and on the other hand, as Roux has
pointed out, all post-embryonic development is ruled and guided by
functioning. It is thus in the long run functioning that brings about
phylogenetic progression, absence of functional activity that causes
phylogenetic regression. This comes about through the transmission of
acquired functional characters, a transmission which Mehnert conceives
to be extraordinarily accurate and complete.

In general Mehnert adopts the functional standpoint of Cuvier, von Baer,
and Roux. His considered judgment as to the phylogenetic value of the
biogenetic law closely resembles that formed by von Baer, for he admits
recapitulation only as regards the single organs, not as regards the
organism as a whole. He has, however, much more sympathy with the law
than either Keibel or Oppel, though he agrees that it cannot be used for
the construction of ancestral trees. But he ascribes to it as a fact of
development considerable importance. The following passage gives a good
summary of his view as to the scope and validity of the law. "The
biogenetic law has not been shaken by the attacks of its opponents. The
assertion is still true that individual organogenesis is exclusively
dependent on phylogeny. But we must not expect to find that all the
stages in the development of the separate organs, which coexisted in any
member of the phylogenetic series, appear _at the same time_ in the
individual ontogeny of the descen