Descendants therefore reach a new goal, either by deviating sooner or
later whilst still on the way towards the form of their parents, or by
passing along this course without deviation, but then, instead of
standing still, advance still farther.
The former mode will have had a predominant action where the posterity
of common ancestors constitutes a group of forms standing upon the same
level in essential features, as the whole of the Amphipoda, Crabs, or
Birds. On the other hand we are led to the assumption of the second mode
of progress, when we seek to deduce from a common original form, animals
some of which agree with young states of others.
In the former case the developmental history of the descendants can only
agree with that of their ancestors up to a certain point at which their
courses separate,--as to their structure in the adult state it will
teach us nothing. In the second case the entire development of the
progenitors is also passed through by the descendants, and, therefore,
so far as the production of a species depends upon this second mode of
progress, the historical development of the species will be mirrored in
its developmental history. In the short period of a few weeks or months,
the changing forms of the embryo and larvae will pass before us, a more
or less complete and more or less true picture of the transformations
through which the species, in the course of untold thousands of years,
has struggled up to its present state.
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