580 
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.)