I am well aware that this doctrine of natural selection, exemplified in the
above imaginary instances, is open to the same objections which were first
urged against Sir Charles Lyell's noble views on "the modern changes of the
earth, as illustrative of geology;" but we now seldom hear the agencies
which we see still at work, spoken of as trifling and insignificant, when
used in explaining the excavation of the deepest valleys or the formation
of long lines of inland cliffs. Natural selection acts only by the
preservation and accumulation of small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection banish the belief of the continued
creation of new organic beings, or of any great and sudden modification in
their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two individuals
must always (with the exception of the curious and not well understood
cases of parthenogenesis) unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless there is reason to
believe that with all hermaphrodites two individuals, either occasionally
or habitually, concur for the reproduction of their kind.
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