If the record
is as fragmentary as many geologists strenuously assert, there is nothing
strange in new forms appearing as if suddenly developed.
Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats, or
the sudden conversion of a Hipparion into a horse, hardly any light is
thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief in
such abrupt changes, embryology enters a strong protest. It is notorious
that the wings of birds and bats, and the legs of horses or other
quadrupeds, are undistinguishable at an early embryonic period, and that
they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter see,
by the progenitors of our existing species having varied after early youth,
and having transmitted their newly-acquired characters to their offspring,
at a corresponding age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species. Hence it is that
existing species during the early stages of their development so often
resemble ancient and extinct forms belonging to the same class. On this
view of the meaning of embryological resemblances, and indeed on any view,
it is incredible that an animal should have undergone such momentous and
abrupt transformations as those above indicated, and yet should not bear
even a trace in its embryonic condition of any sudden modification, every
detail in its structure being developed by insensibly fine steps.
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