accident abnormally resemble certain apes in no less than seven of his
muscles, if there had been no genetic connection between them. On the
other hand, if man is descended from some ape-like creature, no valid
reason can be assigned why certain muscles should not suddenly reappear
after an interval of many thousand generations, in the same manner as
with horses, asses, and mules, dark-coloured stripes suddenly reappear
on the legs, and shoulders, after an interval of hundreds, or more
probably of thousands of generations.
These various cases of reversion are so closely related to those of
rudimentary organs given in the first chapter, that many of them might
have been indifferently introduced either there or here. Thus a human
uterus furnished with cornua may be said to represent, in a rudimentary
condition, the same organ in its normal state in certain mammals. Some
parts which are rudimentary in man, as the os coccyx in both sexes, and
the mammae in the male sex, are always present; whilst others, such as
the supracondyloid foramen, only occasionally appear, and therefore
might have been introduced under the head of reversion. These several
reversionary structures, as well as the strictly rudimentary ones,
reveal the descent of man from some lower form in an unmistakable
manner.
CORRELATED VARIATION.
In man, as in the lower animals, many structures are so intimately
related, that when one part varies so does another, without our being
able, in most cases, to assign any reason. We cannot say whether the
one part governs the other, or whether both are governed by some
earlier developed part. Various monstrosities, as I. Geoffroy
repeatedly insists, are thus intimately connected. Homologous
structures are particularly liable to change together, as we see on the
opposite sides of the body, and in the upper and lower extremities.
Meckel long ago remarked, that when the muscles of the arm depart from
their proper type, they almost always imitate those of the leg; and so,
conversely, with the muscles of the legs. The organs of sight and
hearing, the teeth and hair, the colour of the skin and of the hair,
colour and constitution, are more or less correlated. (55. The
authorities for these several statements are given in my Variation of
Animals under Domestication, vol. ii. pp. 320-335.) Professor
Schaaffhausen first drew attention to the relation apparently existing
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