in his _Descent of Man_ (1871).

In his morphology Darwin was hardly up to date. He does not seem to have
known at first hand the splendid work of the German morphologists, such
as Rathke and Reichert; he pays no attention to the cell-theory, nor to
the germ-layer theory. His sources are, in the main, Geoffroy St
Hilaire, Owen, von Baer, Agassiz, Milne-Edwards, and Huxley.

Perhaps his greatest omission was that he did not give any adequate
treatment of the problem of functional adaptation and the correlation of
parts. It is not too much to say that Darwin not only disregarded these
problems almost entirely, but by his insistence upon ecological
adaptation and upon certain superficial aspects of correlation,
succeeded in giving to the words "adaptation" and "correlation" a new
signification, whereby they lost to a large extent their true and
original functional meaning.

It is true that Darwin himself, as well as his successors, believed that
natural selection was all-powerful to account for the evolution of the
most complicated organs, but it may be questioned whether he realised
all the conditions of the problem of which he thus easily disposed. He
says, rightly, in an important passage, that "It is generally
acknowledged that all organic beings have been formed on two great
laws--Unity of Type, and the Conditions of Existence. By unity of type
is meant that fundamental agreement in structure which we see in organic
beings of the same class, and which is quite independent of their habits
of life. On my theory, unity of type is explained by unity of descent.
The expression of conditions of existence, so often insisted upon by the
illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting _the
varying parts of each being to its organic and inorganic conditions of
life_:[358] or by having adapted them during past periods of time: the
adaptations being aided in many cases by the increased use or disuse of
parts, being affected by the direct action of the external conditions of
life, and subjected in all cases to the several laws of growth and
variation. Hence, in fact, the law of the Conditions of Existence is the
higher law; as it includes, through the inheritance of former variations
and adaptations, that of Unity of Type" (_Origin_, 6th ed., Pop.
Impression, pp. 260-1). It is clear that Darwin took the phrase
"Conditions of Existence" to mean the e