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. They
often become wholly suppressed. When this occurs, they are
nevertheless liable to occasional reappearance through reversion- a
circumstance well worthy of attention.
* I had written a rough copy of this chapter before reading a
valuable paper, "Caratteri rudimentali in ordine all' origine dell'
uomo" (Annuario della Soc. d. Naturalisti, Modena, 1867, p. 81), by G.
Canestrini, to which paper I am considerably indebted. Haeckel has
given admirable discussions on this whole subject, under the title
of "Dysteleology," in his Generelle Morphologie and
Shopfungsgeschichte.
The chief agents in causing organs to become rudimentary seem to
have been disuse at that period of life when the organ is chiefly used
(and this is generally during maturity), and also inheritance at a
corresponding period of life. The term "disuse" does not relate merely
to the lessened action of muscles, but includes a diminished flow of
blood to a part or organ, from being subjected to fewer alternations
of pressure, or from becoming in any way less habitually active.
Rudiments, however, may occur in one sex of those parts which are
normally present in the other sex; and such rudiments, as we shall
hereafter see, have often originated in a way distinct from those here
referred to. In some cases, organs have been reduced by means of
natural selection, from having become injurious to the species under
changed habits of life
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