care, are generally absolutely useless
<was considered>. <These structures,> though sometimes applied to uses
not normal,--which cannot be considered as mere representative parts,
for they are sometimes capable of performing their proper
function,--which are always best developed, and sometimes only
developed, during a very early period of life,--and which are of
admitted high importance in classification,--were shown to be simply
explicable on our theory of common descent.

_Why do we wish to reject the theory of common descent?_

Thus have many general facts, or laws, been included under one
explanation; and the difficulties encountered are those which would
naturally result from our acknowledged ignorance. And why should we not
admit this theory of descent{514}? Can it be shown that organic beings
in a natural state are _all absolutely invariable_? Can it be said that
the _limit of variation_ or the number of varieties capable of being
formed under domestication are known? Can any distinct line be drawn
_between a race and a species_? To these three questions we may
certainly answer in the negative. As long as species were thought to be
divided and defined by an impassable barrier of _sterility_, whilst we
were ignorant of geology, and imagined that the _world was of short
duration_, and the number of its past inhabitants few, we were justified
in assuming individual creations, or in saying with Whewell that the
beginnings of all things are hidden from man. Why then do we feel so
strong an inclination to reject this theory--especially when the actual
case of any two species, or even of any two races, is adduced--and one
is asked, have these two originally descended from the same parent womb?
I believe it is because we are always slow in admitting any great
change of which we do not see the intermediate steps. The mind cannot
grasp the full meaning of the term of a million or hundred million
years, and cannot consequently add up and perceive the full effects of
small successive variations accumulated during almost infinitely many
generations. The difficulty is the same with that which, with most
geologists, it has taken long years to remove, as when Lyell propounded
that great valleys{515} were hollowed out [and long lines of inland
cliffs had been formed] by the slow action of the waves of the sea. A
man may long view a grand precipice without actually believing, though
he may not deny it, that thousands of feet i