204 COMPARATIVE ANATOMY 



come to them of necessity, but as an accident, through de- 

 privation of food or violence. But we know too little of the 

 life-history of Amoeba to allow of our making any positive 

 assertion about it, and the same may be said of most other 

 Protozoa. We must be content at present with the positive 

 evidence afforded by the life-histories of Paramecium and 

 allied forms, which demonstrate that the organism would wear 

 out and perish were it not refreshed, rejuvenated from time to 

 time, by the act of conjugation. How it is that conjugation 

 rejuvenates the enfeebled organisation we cannot say. 



But it must be remembered, whilst we admit decay and 

 death as the natural accompaniments of existence, that the 

 life-stuff, protoplasm in its widest sense, is immortal. All the 

 evidence at our disposal forces us to believe that living matter 

 can only be born from living matter. Organisms are not bred, 

 as was once believed, from slime and mud, but only from other 

 organisms, and all that now lives on the earth is descended 

 from that original life-stuff whose origin will ever be a mystery 

 to us. 



