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relatives, conjugation had not occurred. The result, corroborated in
other cases, was striking. The whole family became exhausted, small
and senile; they ceased to divide or even to feed; their nuclei under
went a strange degeneration ; they began to die. But individuals,
removed before the process had gone too far, were observed to
conjugate with unrelated forms, and to live on. The inference was
obvious. Conjugation in these Infusorians is of little moment to any
two individuals ; during long periods it need never occur, but it is
essential to the continued life of the species. ‘ It is a necessary
condition of their eternal youth. 5 ”—(J. Arthur Thomson. The Study
of Animal Life. p. 214.)
It is quite clear that conjugation in this case is necessary
simply for the renewal of vital energy; and if, in spite of
such conjugation, the species remain unmodified, there is
no reason for assuming that Natural Selection has inter
posed in any way.
In other cases of conjugation, there may be an output
of individual differences ; but such differences are always
occurring in connection with species which remain un
modified ; and it is a large assumption to take for granted
that Natural Selection has been at work simply because
one of its conditions has occurred ; especially when that
condition is one inevitable in connection with sexual
reproduction.
Mr. Spencer, in pursuing his argument, proceeds to
point out the conditions under which Natural Selection
becomes a predominating factor.
“When, along with the growing multiplication in forms of life,
conflict and competition became continually more active, fortuitous
variations of structure, of no account in the converse with the
medium, became of much account in the struggle with enemies and
competitors, and Natural Selection of such variations became the
predominant factor. Especially throughout the plant world its action
appears to have been immensely the most important, and throughout
that large part of the animal world characterised by relative inactivity,
the survival of individuals that had varied in favourable ways must all
along have been the chief cause of the divergence of species and the
occasional production of higher ones.”—{ftp. 73-4.)