It will be enough for us merely to establish
the fact that the one-toed horse is an evolved descendant of a
primitive five-toed mammal, through the adaptation of its foot to
running on firm ground, its teeth and neck to feeding on grasses,
and so on.
On the other hand, the facts we have already seen seem to justify
the attitude of compromise I adopted in regard to the Mutationist
theory. It would be an advantage in many ways if we could believe
that new species arose by sudden and large variations (mutations)
of the young from the parental type. In the case of many organs
and habits it is extremely difficult to see how a gradual
development, by a slow accentuation of small variations, is
possible. When we further find that experimenters on living
species can bring about such mutations, and when we reflect that
there must have been acute disturbances in the surroundings of
animals and plants sometimes, we are disposed to think that many
a new species may have arisen in this way. On the other hand,
while the palaeontological record can never prove that a species
arose by mutations, it does sometimes show that species arise by
very gradual modification.
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