THE CONTINUITY OF LIFE 7 



it seems that the reproductive force becomes lessened, causing 

 longer pauses between successive fissions, and ultimately the 

 death of the organisms. It is at this time, when the fissions 

 are farther between and carried on with less activity, that 

 conjugation appears. This consists typically of the temporary 

 fusion of two individuals, during which there is a mutual inter- 

 change of certain of the elements of the nuclei. When this 

 is accomplished the two individuals, or gametes, as they are 

 here termed, separate, and begin anew a fresh series of fissions 

 as at first. 



The purpose of the process thus seems to be something like 

 a rejuvenescence, by means of which the reproductive activity 

 may be renewed; yet, that the action is chemical rather than 

 physiological is suggested by experiments in which a similar 

 increase of activity, taking the place of conjugation, may be 

 induced by the addition of food-substances like beef broth to 

 the water containing the species under investigation. 



In many cases the process of conjugation is rendered more 

 complicated by the introduction of two sorts of individuals, 

 macro- and micro-gametes, which are evidently produced for 

 this especial purpose by a variation in the usual course of the 

 fission process. In this case the two usually unite perma- 

 nently and form a zygote, which becomes thus endowed with 

 special reproductive activity. [Fig. 4.] 



In multicellular organisms the matter becomes still more 

 complicated, but is essentially the same so far as concerns pro- 

 toplasmic continuity. Here only certain cells, which are called 

 germ-cells , act as gametes and conjugate, producing the new 

 organisms by their repeated divisions, while the remainder, 

 often vastly preponderating over the former in actual bulk, 

 build up a body or so ma, which forms a shelter and protection 

 for the germ cells. Somata possess a high degree of adapt- 

 ability to external conditions, and become modified to fit them, 

 so that in this way they and the germ-cells contained within 

 them may come to be developed in places and under circum- 

 stances where otherwise they could not possibly exist. 



In this way all animal and plant forms have been produced, 



