Internalism and Laws of Form 337
bones arrest their development and shorten them, and this affects the position of
the subsequently developed molar teeth" (1868, vol. 2, p. 321).
Darwin then inserts a new category, strangely overlooked in 1859 (a testi-
mony, perhaps, to his subsidiary concern for the general subject of structural
constraint)—allometric effects of change in size, either of the whole body, or of
parts:
Another simple case of correlation is that with the increased or decreased
dimensions of the whole body, or of any particular part, certain organs are
increased or diminished in number, or are otherwise modified. Thus
pigeon-fanciers have gone on selecting pouters for length of body, and we
have seen that their vertebrae are generally increased in number, and their
ribs in breadth.... In Germany it has been observed that the period of
gestation is longer in large-sized than in small-sized breeds of cattle. With
our highly improved animals of all kinds the period of maturity has
advanced... and, in correspondence with this, the teeth are now developed
earlier than formerly, so that, to the surprise of agriculturists, the ancient
rules for judging the age of an animal by the state of its teeth are no longer
trustworthy (1868, vol. 2, p. 321).As in 1859, Darwin devotes most attention to correlated variability in
homologous structures. In part, this preference can claim a methodological basis,
for Darwin finds less mystery in the bonding or joint variation of homologs than in
most other cases of structural constraint:
In many cases of slight deviations of structure as well as of grave
monstrosities, we cannot even conjecture what is the nature of the bond of
connection. But between homologous parts—between the fore and hind
limbs—between the hair, hooves, horns, and teeth—we can see that parts
which are closely similar during their early development, and which are
exposed to similar conditions, would be liable to be modified in the same
manner. Homologous parts, from having the same nature, are apt to blend
together, and, when many exist, to vary in number (1868, vol. 2, pp. 419-
420).Amidst a plethora of interesting examples, Darwin cites pigeons that develop
feathers and incipient wing-like membranes on portions of the foot corresponding
to the position of wings on the forelimbs: "In feather-footed pigeons, not only does
the exterior surface support a row of long feathers, like wing feathers, but the very
same digits which in the wing are completely united by skin become partially
united by skin in the feet; and thus by the law of the correlated variation of
homologous parts we can understand the curious connection of feathered legs and
membrane between the two outer toes" (1868, vol. 2, p. 323).
Citing the theory of vertebral archetypes again, Darwin tries to correlate
variation in head and limb bones, while acknowledging that not all biologists
accept the idea: "If those naturalists are correct who maintain that the jawbones are
homologous with the limb bones, then we can understand why the