Here natural selection has probably
come into play, as well as in adapting the general shape of the body of
these fishes, and many other peculiarities, to their habits of life. We
should keep in mind, as I have before insisted, that the inherited effects
of the increased use of parts, and perhaps of their disuse, will be
strengthened by natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those individuals which
inherit in the highest degree the effects of the increased and beneficial
use of any part. How much to attribute in each particular case to the
effects of use, and how much to natural selection, it seems impossible to
decide.
I may give another instance of a structure which apparently owes its origin
exclusively to use or habit. The extremity of the tail in some American
monkeys has been converted into a wonderfully perfect prehensile organ, and
serves as a fifth hand. A reviewer, who agrees with Mr. Mivart in every
detail, remarks on this structure: "It is impossible to believe that in
any number of ages the first slight incipient tendency to grasp could
preserve the lives of the individuals possessing it, or favour their chance
of having and of rearing offspring." But there is no necessity for any
such belief. Habit, and this almost implies that some benefit great or
small is thus derived, would in all probability suffice for the work.
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