arters will be
progressively fitted to the changed structure of the fore-quarters: all
the appliances for nutrition and innervation being at the same time
progressively fitted to both. But in the absence of this inheritance of
functionally-produced modifications, there is no seeing how the required
re-adjustments can be made.

* * * * *

Yet a third class of difficulties stands in the way of the belief that
the natural selection of useful variations is the sole factor of organic
evolution. This class of difficulties, already pointed out in Sec. 166 of
the _Principles of Biology_, I cannot more clearly set forth than in the
words there used. Hence I may perhaps be excused for here quoting them.

"Where the life is comparatively simple, or where surrounding
circumstances render some one function supremely important, the
survival of the fittest may readily bring about the appropriate
structural change, without any aid from the transmission of
functionally-acquired modifications. But in proportion as the life
grows complex--in proportion as a healthy existence cannot be
secured by a large endowment of some one power, but demands many
powers; in the same proportion do there arise obstacles to the
increase of any particular power, by 'the preservation of favoured
races in the struggle for life.' As fast as the faculties are
multiplied, so fast does it become possible for the several members
of a species to have various kinds of superiorities over one
another. While one saves its life by higher speed, another does the
like by clearer vision, another by keener scent, another by quicker
hearing, another by greater strength, another by unusual power of
enduring cold or hunger, another by special sagacity, another by
special timidity, another by special courage; and others by other
bodily and mental attributes. Now it is unquestionably true that,
other things equal, each of these attributes, giving its possessor
an extra chance of life, is likely to be transmitted to posterity.
But there seems no reason to suppose that it will be increased in
subsequent generations by natural selection. That it may be thus
increased, the individuals not possessing more than average
endowments of it, must be more frequently killed off than
individuals highly endowed with it; and this can happe