360 THE president's address. 



are not sufficiently cooled to permit of the existence on them of 

 living beings. 



How would such a germ be enabled to leave our earth and 

 embark on a voyage in space 1 Arrheniiis supposes that a living 

 particle sufficiently minute might get carried by winds and 

 atmospheric disturbances up into the higher layers of our 

 atmosphere until the radiation -pressure of the sun's rays would 

 be sufficient to counterbalance the attraction of gravity, and 

 that then it would be wafted out into space. An organism 

 detached from our earth by the radiation-pressure of the sun 

 would, according to Arrhenius, cross the orbit of Mars after 

 20 days, of Jupiter after 80 days, of Neptune after 14 months, 

 and the nearest solar system, Alpha Centauri, could be reached 

 in about 9,000 years. In order to undergo the strongest 

 influence of solar radiation, an organism must have a diameter 

 of 016 fx that is to say, y-geV-o ^^ ^^ inch. 



Such, in its main features, is the doctrine of panspermia 

 advocated by Arrhenius. We may now examine it a little 

 closer with regard to both its details and its consequences. I am 

 not competent to criticise it from the point of view of its 

 physical aspects that is to say, with regard to the theory of 

 transportation of particles by radiation-pressure ; that must be 

 left to the physicists and astronomers, by whom, I believe, it is 

 accepted. I can only deal with it as a naturalist. Our first 

 inquiry is naturally concerning the living germs themselves. 



As no one has as yet seen a germ of life at its first arrival on 

 our earth, we can only consider what type of organism amongst 

 those known to us would be capable of quitting the confines of 

 our atmosphere and embarking on a voyage in space. It is clear, 

 I think, that no Protozoan cyst, no ordinary seed or spore of any 

 plant, could be carried off our earth by radiation-pressure, 

 because such organisms would be many times too big to satisfy 

 the physical requirements of the theory. Still more is this true 

 of the ordinary visible forms of animal life ; the familiar 

 expression " raining cats and dogs " cannot be taken literally, 

 and the Arrhenian theory offers no prospect that any of us will 

 ever be able to pay a visit to Mars or any other distant world. 

 Even most bacteria would be far too large to be carried off the 

 earth, though possibly some of the minuter forms of micrococci, 

 in the state of spores, might undertake the journey. But I am 



