limb, may be increased as a whole by
the simultaneous due increase of its co-operative parts; since if, while
it is growing, the channels of supply bring to the limb an unusual
quantity of blood, there will naturally result a proportionately greater
size of all its components--bones, muscles, arteries, veins, &c. But
though in cases like this, the co-operative parts forming some large
complex part may be expected to vary together, nothing implies that they
necessarily do so; and we have proof that in various cases, even when
closely united, they do not do so. An example is furnished by those
blind crabs named in the _Origin of Species_ which inhabit certain dark
caves of Kentucky, and which, though they have lost their eyes, have
not lost the foot-stalks which carried their eyes. In describing the
varieties which have been produced by pigeon-fanciers, Mr. Darwin notes
the fact that along with changes in length of beak produced by
selection, there have not gone proportionate changes in length of
tongue. Take again the case of teeth and jaws. In mankind these have not
varied together. During civilization the jaws have decreased, but the
teeth have not decreased in proportion; and hence that prevalent
crowding of them, often remedied in childhood by extraction of some, and
in other cases causing that imperfect development which is followed by
early decay. But the absence of proportionate variation in co-operative
parts that are close together, and are even bound up in the same mass,
is best seen in those varieties of dogs named above as illustrating the
inherited effects of disuse. We see in them, as we see in the human
race, that diminution in the jaws has not been accompanied by
corresponding diminution in the teeth. In the catalogue of the College
of Surgeons Museum, there is appended to the entry which identifies a
Blenheim Spaniel's skull, the words--"the teeth are closely crowded
together," and to the entry concerning the skull of a King Charles's
Spaniel the words--"the teeth are closely packed, p. 3, is placed quite
transversely to the axis of the skull." It is further noteworthy that in
a case where there is no diminished use of the jaws, but where they have
been shortened by selection, a like want of concomitant variation is
manifested: the case being that of the bull-dog, in the upper jaw of
which also, "the premolars ... are excessively crowded, and placed
obliquely or even transversely to the long axis of the s