AND ITS INHABITANTS 97 



that certain types of bacteria can successfully endure a 

 temperature of nearly 200 C. for six months, and about 

 250 C. for shorter periods, that is, a temperature con- 

 siderably lower than that at which any chemical reactions 

 are known to occur; and again the spores of other bacteria 

 can withstand a temperature as high as 120 C. for a short 

 time. The cysts of some relatively highly specialized protozoa 

 can retain their vitality for at least fifty years, and the seeds 

 of the higher plants, particularly those with a thick and im- 

 permeable integument, have been found to retain the power of 

 germination for nearly a century, though the statement that 

 grain from Egyptian tombs still maintains its power of growth 

 has been disproved. The ability to continue dormant life 

 clearly seems to depend to a large extent on the presence 

 of a dry enclosing membrane, since under these conditions the 

 entrance of gases is impossible and the chemical processes 

 within are at a minimum. Thus it is apparent that certain 

 organisms may survive unfavorable conditions for a long 

 period, and we have no reason for believing that the limits of 

 endurance of other forms do not greatly exceed that of the 

 few which have thus far been studied. 



On the other hand, the exigencies to which living matter 

 would be exposed when started on its interstellar journey are 

 not inconsequential. Meteors in their fall through the earth's 

 atmosphere become incandescent and, if they are the vehicle 

 of transfer, it would only be conceivable for life to survive 

 far below the surface where the temperature is lower. To 

 avoid this and other difficulties it has been suggested by 

 Arrhenius that the radiation pressure of light is sufficient to 

 overcome the attraction of gravitation for particles of the 

 extraordinary minuteness of some of the lowest forms of life, 

 and that isolated germs might make the journey to the earth. 

 But on the assumption that such an individual organism were 

 forced out into space by the mechanical pressure of the light 



