e principal organs might be moulded by mere
local inequalities of growth--the ventricles of the brain, for instance,
the alimentary canal, the heart--and he further succeeded in imitating
the formation of these organs by folding, pinching, and cutting
india-rubber tubes and plates in various ways."[469]

But Roux was undoubtedly the first to make a systematic survey of the
problems to be solved and to work out an organised method of attack. His
earliest work deals with the important problem of functional
adaptation--its importance to the organism, and its possible mechanistic
explanation. The first paper[470] was a study of the branching and
distribution of the arteries in the human body (1878), and a second
paper on the same subject followed in 1879.[471]

In these papers Roux showed how the development of the blood-vascular
system was largely determined by direct adaptation to functional
requirements, and he inferred the existence in the vascular tissues of
certain vital properties, in virtue of which the functional adaptation
of the blood-vessels came about. Thus the intima or inner lining must
possess the faculty of so reacting to the friction set up by the
blood-current as to oppose the least possible resistance to its flow;
the muscular coats must react to increased pressure by growing thicker,
and so on.

These papers were followed in 1881 by his well-known book, _Der Kampf
der Theile im Organismus_, which contained the working-out of his
mechanistic explanation of functional adaptation, and most of the
elements of his general "causal-analytical" theory of form production.
The significance of the book was popularly considered at the time to lie
in its supposed application of the selection idea to the explanation of
the internal adaptedness of animal structure--in the theory of "cellular
selection," and the book owed its success to its fitting in so well with
the prevalent Darwinism of the day. But its real importance, as a big
step towards causal morphology, was naturally not so fully appreciated.

During the next few years Roux continued his studies on functional
adaptation,[472] and at the same time made a new departure by
inaugurating, almost contemporaneously with the physiologist Pflueger,
the study of experimental embryology. Isolated observations had
previously been made upon the development of single blastomeres or parts
of blastulae, by Haeckel and Chun for instance,[473] but Roux[474] and
Pflueger[4