188 SCIENCE PROGRESS 



two, and by this simple process a new generation is formed. 

 Clearly, as Weissmann first pointed out, in such cases death 

 intrudes only in the guise of accident. 



Now, the conditions of life of these simplest forms are by 

 no means simple. Make an infusion of hay in boiling water, 

 and let it cool. In the course of a day or so it will be found 

 to be swarming with rod-like bacteria {Bacterium subtilis), engaged 

 in feeding on the organic matter dissolved out of the hay. A 

 few days later numbers of an actively mobile slipper-shaped 

 animal, called Paramecium caudatum, make their appearance to 

 actively swallow and digest the bacteria ; and so the round goes 

 on. The bacteria and the Paramcecia alike have developed from 

 wind-carried spores. Therefore in the natural life of these 

 creatures are periods of physiological activity alternating with 

 periods when life seems to be completely dormant — periods 

 which follow one another according to no regular sequence, 

 but in consequence of chance rainfall and of drought, when the 

 inhabitants of the dried-up pool are caught up and carried away 

 as dust. 



Watch any chance collection of Paramcecia, and individuals 

 will be seen not only to divide, but occasionally to fuse. Two 

 individuals swim together, adhere closely, and effect an extensive 

 interchange of substance. This is the process of conjugation : 

 it is the first beginning of sexual reproduction. It is followed 

 by increased physiological activity, increased rate of growth 

 and of multiplication. If we could follow this mating process 

 fully, if our imagination could grasp the events which lead 

 up to it and the effects which follow, we should see in it the 

 response of life to the flux of cosmical energy, just as the 

 oscillations of a particle in Brownian movement are the response 

 to the flux of electrical potential. This is no careless phrase : 

 it is sober truth, for the air currents carry and mix spores 

 from far-distant places which have, therefore, had different life- 

 histories. They have lived in waters draining different kinds 

 of soil, and therefore chemically different, the balance of sun- 

 shine and shade has been different, and the wind capriciously 

 sows these spores from north, south, east, and west in the pot 

 of hay tea. There they become active, and mate in conjugation, 

 but not fortuitously. Guided by chemiotaxis, unlikes meet and 

 fuse, just as do unlike cells when an ovum and a spermatozoon 

 fuse, and the fusion of a pair of unlike individuals results in 



