My reasons for doubting whether natural species have changed as abruptly as
have occasionally domestic races, and for entirely disbelieving that they
have changed in the wonderful manner indicated by Mr. Mivart, are as
follows. According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather long
intervals of time. If such occurred under nature, they would be liable, as
formerly explained, to be lost by accidental causes of destruction and by
subsequent intercrossing; and so it is known to be under domestication,
unless abrupt variations of this kind are specially preserved and separated
by the care of man. Hence, in order that a new species should suddenly
appear in the manner supposed by Mr. Mivart, it is almost necessary to
believe, in opposition to all analogy, that several wonderfully changed
individuals appeared simultaneously within the same district. This
difficulty, as in the case of unconscious selection by man, is avoided on
the theory of gradual evolution, through the preservation of a large number
of individuals, which varied more or less in any favourable direction, and
of the destruction of a large number which varied in an opposite manner.
That many species have been evolved in an extremely gradual manner, there
can hardly be a doubt. The species and even the genera of many large
natural families are so closely allied together that it is difficult to
distinguish not a few of them.
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