kull."[41]

If, then, in cases where we can test it, we find no concomitant
variation in co-operative parts that are near together--if we do not
find it in parts which, though belonging to different tissues, are so
closely united as teeth and jaws--if we do not find it even when the
co-operative parts are not only closely united, but are formed out of
the same tissue, like the crab's eye and its peduncle; what shall we say
of co-operative parts which, besides being composed of different
tissues, are remote from one another? Not only are we forbidden to
assume that they vary together, but we are warranted in asserting that
they can have no tendency to vary together. And what are the
implications in cases where increase of a structure can be of no service
unless there is concomitant increase in many distant structures, which
have to join it in performing the action for which it is useful?

As far back as 1864 (_Principles of Biology_, Sec. 166) I named in
illustration an animal carrying heavy horns--the extinct Irish elk; and
indicated the many changes in bones, muscles, blood-vessels, nerves,
composing the fore-part of the body, which would be required to make an
increment of size in such horns advantageous. Here let me take another
instance--that of the giraffe: an instance which I take partly because,
in the sixth edition of the _Origin of Species_, issued in 1872, Mr.
Darwin has referred to this animal when effectually disposing of certain
arguments urged against his hypothesis. He there says:--

"In order that an animal should acquire some structure specially
and largely developed, it is almost indispensable that several
other parts should be modified and co-adapted. Although every part
of the body varies slightly, it does not follow that the necessary
parts should always vary in the right direction and to the right
degree" (p. 179).

And in the summary of the chapter, he remarks concerning the adjustments
in the same quadruped, that "the prolonged use of all the parts together
with inheritance will have aided in an important manner in their
co-ordination" (p. 199): a remark probably having reference chiefly to
the increased massiveness of the lower part of the neck; the increased
size and strength of the thorax required to bear the additional burden;
and the increased strength of the fore-legs required to carry the
greater weight of both. But now I think that further consideration
s