Sex as an Adaptation 441 
What we really need to know is whether or not the sexual 
method was once acquired, because it was an advantage 
to a particular organism, or to the species to reproduce in 
this way. It is assumed by many writers that this was the 
case, but whether they have sufficient ground for forming 
such an opinion is our chief concern here. On the other 
hand, it is conceivable, at least, that if the sexual method 
once became established, it might continue without respect to 
any superiority it gave over other methods, and might finally 
become a necessary condition for the propagation of particu- 
lar species. Thus the method would become essential to 
propagation without respect to whether the species lost more 
than it gained. Whichever way the balance should turn, it 
might make little difference, so long as the species was still 
able to propagate itself. 
Brooks made the interesting and ingenious suggestion that 
the separation of the sexes has been brought about as a sort 
of specialization of the individuals in two directions. The 
male cells are supposed to accumulate the newly acquired 
characters, and represent, therefore, the progressive element 
in evolution. The female cells are the conservative element, 
holding on to what has been gained in the past. It does not 
seem probable, in the light of more recent work, that this 
is the function of the two sexes, and it is unlikely that we 
could account for the origin of the two sexes through the 
supposed advantage that such a specialization might bring 
about. A number of writers, Galton, Van Beneden, Biitschli, 
Maupas, and others, have looked at the process of sexual re- 
production as a sort of renewal of youth, or rejuvenescence 
of the individuals. There is certainly a good deal in the 
process to suggest that something of this sort takes place, 
although we must be on our guard against assuming that 
the rejuvenescence is anything more than the fulfilment of a 
necessary stage in the life history. Weismann has ridiculed 
this suggestion on the ground that it is inconceivable that 
