ll;" and the
cavities between the bony plates "are not so deep, nor do they extend
beyond the frontals."[835]

* * * * *

It may be well here to pause and observe how the effects of correlated
variability, of the increased use of parts, and of the accumulation through
natural selection of so-called spontaneous variations, are in many cases
inextricably commingled. We may borrow an illustration from Mr. Herbert
Spencer, who remarks that, when the Irish elk acquired its gigantic horns,
weighing above one hundred pounds, numerous co-ordinated {334} changes of
structure would have been indispensable,--namely, a thickened skull to
carry the horns; strengthened cervical vertebrae, with strengthened
ligaments; enlarged dorsal vertebrae to support the neck, with powerful
fore-legs and feet; all these parts being supplied with proper muscles,
blood-vessels, and nerves. How then could these admirably co-ordinated
modifications of structure have been acquired? According to the doctrine
which I maintain, the horns of the male elk were slowly gained through
sexual selection,--that is, by the best-armed males conquering the
worse-armed, and leaving a greater number of descendants. But it is not at
all necessary that the several parts of the body should have simultaneously
varied. Each stag presents individual differences, and in the same district
those which had slightly heavier horns, or stronger necks, or stronger
bodies, or were the most courageous, would secure the greater number of
does, and consequently leave a greater number of offspring. The offspring
would inherit, in a greater or less degree, these same qualities, would
occasionally intercross with each other, or with other individuals varying
in some favourable manner; and of their offspring, those which were the
best endowed in any respect would continue multiplying; and so onwards,
always progressing, sometimes in one direction, and sometimes in another,
towards the present excellently co-ordinated structure of the male elk. To
make this clear, let us reflect on the probable steps, as shown in the
twentieth chapter, by which our race and dray-horses have arrived at their
present state of excellence; if we could view the whole series of
intermediate forms between one of these animals and an early unimproved
progenitor, we should behold a vast number of animals, not equally improved
in each generation throughout their entire structure, but so