* 54 



THE EVOLUTION OF SEX. 



multiplication by division can never, — not merely in certain condi- 

 tions, — but never continue unlimitedly. This can not, however, be 

 proved, just as little as the reverse." But Weismann must surely admit 

 that the demonstration of even some cases where species, normally 

 reproducing asexually, come to an absolute standstill if conjugation 

 be prevented, would give considerable strength to the interpretation 

 of fertilization as rejuvenescence. Such cases have, happily, come to 

 hand, as we shall now see. 



We have already referred to Maupas's proof of true sexual union in 

 ciliated infusorians. By an elaborate process of nuclear division, 

 disruption, elimination, interchange, union, and reconstruction, two 

 ' ' slipper animalcules ' ' fertilize one another. What is the meaning of 

 all this? 



Each infusorian, after conjugation, proceeds to divide, but the 

 results are to all appearance the same as it previously produced. 

 There is no special sexually produced generation. 



It has been often alleged that the subsequent dividing is accelerated 

 by conjugation ; but Maupas finds that this is not so. The reverse in 

 fact is true, — it is a loss of time. While a pair of infusorians 

 {Onychodr omits grandis) were indulging in a single conjugation, 

 another had become, by ordinary asexual division, the ancestor of 

 from forty thousand to fifty thousand individuals. 



Moreover, the intense internal change preparatory to fertilization, 

 and the general inertia during subsequent reconstruction, not only 

 involve loss of time, but expose the infusorians to great risk. It 

 seems then like a condition of danger and death rather than of multi- 

 plication and birth. 



The riddle was, in part at least, solved by a long series of care- 

 ful observations. In November, 1885, M. Maupas isolated an infuso- 

 rian (Stylonichia pustulata), and observed its generations till March, 

 1886. By that time there had been two hundred and fifteen genera- 

 tions produced by ordinary division, and since these lowly organisms 

 do not conjugate with near relatives, there had, of course, been no 

 sexual union. 



What was the result? At the date referred to, the family was 

 observed to have exhausted itself. They were not old exactly, but 

 they were being born old. The asexual division came to a stand- 

 still, and the powers of nutrition were also lost. 



Meanwhile, however, several of the individuals, before the genera- 

 tions had exhausted themselves, had been removed to another basin, 

 where they conjugated with unrelated forms of the same species. One 

 of these was again isolated, and watched for five months. The usual 

 richness of successive generations occurred; members removed at dif- 



