by G. Wolff in 1895.[522] The
experiment was later repeated and confirmed by Fischel and other
workers. Wolff drew from this and other facts the conclusion that the
organism possesses a faculty of "primary purposiveness" which cannot
have arisen through natural selection.[523] And, as is well known, Driesch
derived one of his most powerful arguments in favour of vitalism from
the extraordinary regenerative processes shown by _Tubularia_ and
_Clavellina_ in the course of which the organism actually demolishes and
rebuilds a part or the whole of its structure. But under the influence
of physiologists like Loeb many workers held fast to materialistic
methods and conceptions.

The great variety of regulative response of which the organism showed
itself capable made it very difficult for the morphologist to uphold the
generalisations which he had drawn from the facts of normal undisturbed
development. The germ-layer theory was found inadequate to the new
facts, and many reverted to the older criterion of homology based on
destiny rather than origin. The trend of opinion was to reject the
ontogenetic criterion of homology, and to refuse any morphological or
phylogenetic value to the germ-layers.[524]

The biogenetic law came more and more into disfavour, as the developing
organism more and more showed itself to be capable of throwing off the
dead-weight of the past, and working out its own salvation upon original
and individual lines.[525] A. Giard in particular called attention to a
remarkable group of facts which went to show that embryos or larvae of
the same or closely allied species might develop in most dissimilar ways
according to the conditions in which they found themselves.[526] His
classical case of "poecilogeny" was that of the shrimp _Palaemonetes
varians_, the fresh-water form of which develops in an entirely
different way from the salt-water form.

Experimental workers indeed were inclined to rule the law out of
account, to disregard completely the historical element in development,
and this was perhaps the chief weakness of the neo-vitalist systems
which took their origin in this experimental work.

From the side also of descriptive morphology the biogenetic law
underwent a critical revision. It was studied as a fact of embryology
and without phylogenetic bias by men like Oppel, Keibel, Mehnert, O.
Hertwig and Vialleton,[527] and they arrived at a critical estimate of it
very similar to that of von Baer.