196 MANUAL OF ELEMENTARY ZOOLOGY 



that conjugation properly so called is not needed for the 

 revitalising of the germs, but has some other function. If 

 the " conjugation " of Paramecium has indeed a revitalising 

 effect, this may be due to the replacement of an effete 

 meganucleus, and not to the true conjugation — the union 

 of micronuclei. That seems the more likely from an exceed- 

 ingly interesting observation made upon a culture in which 

 thousands of healthy generations succeeded one another 

 without conjugation. Here it was found that at intervals 

 the meganucleus was destroyed and replaced from the 

 unconjugated micronucleus. This view is supported by 

 other cases where the germ-substance is seen to keep its 

 vitality without conjugation, such as that of Amceba proteus, 

 in which there is no distinction between germ-substance 

 and body-substance, and life can go on for many generations 

 (for ever, so far as is at present known) without conjugation, 

 and again the parthenogenesis (p. 190) of certain higher 

 animals, when ova, exactly like those which in other animals 

 are destined to conjugate, are separated from the body and 

 develop without conjugation for an indefinite number of 

 generations. To sum up : it is doubtful whether " con- 

 jugation " of the members of a depressed culture can revive 

 it ; if it does so, that is an abnormal function ; and in any 

 case any rejuvenating effect of this process is probably due 

 to the replacement of the meganucleus. The reason of 

 the union of the germs, in these as in other circumstances, 

 is as obscure as ever. 



(3) Another class of theories as to the meaning of this 

 process regards it as combining the hereditary characters of 

 different individuals. It is well known that the individuals 

 of a species of animal differ from one another in respect 

 of each of their characters. Some will be larger, more 

 brightly coloured, more intelligent, and so forth, than 

 others. It is also certain that many of these differences 

 are inherited ; the children of a tall man will, for instance, 

 be on the average taller than those of a short man. These 

 hereditary tendencies must in some way be conveyed by 

 the gametes, but the benefit conferred by the combination 

 which is brought about in the zygote is the subject of more 

 than one theory, (a) One result of the union of gametes 

 is to produce offspring which are not exactly like either 



