I02 PROTOZOA. 



That there is in these cases a mutual fertilisation of some 

 sort we cannot doubt. But the precise interpretation of 

 the process is to some extent a matter of mere opinion. 

 We may regard it as a mutual rejuvenescence, each unit 

 supplying some substances or qualities which the other 

 lacks ; or we may regard it rather as a process by which the 

 average character of the species is sustained, peculiarities 

 or pathological variations of one individual being counter- 

 acted by other characters in the neighbour (apparently no 

 near relation) with which it conjugates. But the researches 

 of M. Maupas have thrown considerable light on the facts, 

 and some of his results I shall now summarise. 



It has been often alleged that the subsequent dividing is 

 accelerated by conjugation ; but Maupas finds that this is 

 by no means the case. The reverse in fact is true. While 

 a pair of Infusorians {Onychodromus grandis) were engaged 

 in conjugation, a single individual had, by ordinary asexual 

 division, given rise to a family of from forty thousand to 

 fifty thousand individuals. Moreover, the intense internal 

 change preparatory to fertilisation, and the general inertia 

 during subsequent reconstruction, not only involve loss of 

 time, but expose the Infusorians to great risk. Conjug- 

 ation seems to involve danger and death rather than to 

 conduce to multiplication and birth. 



The riddle was, in part at least, solved by a long series ot 

 careful observations. In November 1885, M. Maupas iso- 

 lated an infusorian {Stylonichia pustulatd) and observed its 

 generations till March 1886. By that time there had been 

 two hundred and fifteen generations 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. The members 

 were being born old and debilitated. The asexual division 

 came to a standstill, and the powers of nutrition were 

 lost. 



Meanwhile, before the generations had exhausted them- 

 selves, several of the individuals had been restored to their 

 natural conditions, where they conjugated with unrelated 

 forms of the same species. One of these was again isolated, 



