porary changes are sharply to be distinguished all permanent
alterations which first appear in perceptible fashion through
oft-repeated or long-continued, enhanced functional activity. These
produce a new and lasting internal equilibrium of the organ, consisting
in an insertion of new molecules or a rearrangement of old. For this
reason they outlast the periods of functional form-change, or, if as in
the case of the muscles they themselves alter during functional
activity, they regain their state when the organ ceases to function" (p.
72, 1910). "Oft-repeated exercise or heightened exercise of the specific
functions, or repeated action of the functional stimuli which determine
them, produces, as we have said before, true form-changes as a
by-product. These are of two kinds. In so far as these form-changes
facilitate the repetition of the specific functions, I have called them
_functional adaptations_.... Such as do not improve the functioning of
the organ are indeed by-products of functioning, but without adaptive
character; they do not belong to the class of functional adaptations at
all" (p. 75, 1910).

We may now enquire in what way functional adaptations can arise as
by-products of functioning.

It is clear that natural selection in the sense of individual or
"personal" selection cannot adequately explain the origin of functional
structure and the functional harmony of structure, for thousands of
cells would have to vary together in a purposive way before any real
advantage could be gained in the struggle for existence, and it is in
the highest degree unlikely that this should come about by chance
variation.[486] The development of purposive internal structure is only to
be explained by the properties of the tissues concerned.

In illustration and proof of the statement that functional adaptation is
due to the properties of the tissues we may adduce the development and
regulation of the blood-vascular system, which has been thoroughly
studied from this point of view by Roux and Oppel (1910).

It appears that only the very first rudiments of the vascular system are
laid down in the short first period of automatic non-functional
development. All the subsequent growth and differentiation of the
blood-vessels falls into the second period, and is due wholly or in
great part to direct functional adaptation to the requirements of the
tissues. Thus from the rudiments formed in the first period there sprout
out the defin