THE PRESIDENT'S ADDRESS. 265 



these invisible germs is enigmatical in the highest degree. They 

 are supposed to be propelled through space by the pressure of 

 the radiant energy streaming from the sun and it has indeed 

 been demonstrated that very minute particles can be propelled in 

 this way by rays of light. It has been objected to this view 

 that no organisms could withstand the intense cold of inter- 

 planetary space, but we know that living organisms withstand 

 low temperatures much better than they withstand high ones, 

 and there appears to be no known minimum at which all life 

 is necessarily destroyed. A more serious objection is to be found 

 in what is known of the fatal effects of ultra-violet light rays 

 upon micro-organisms. At the surface of the earth such 

 organisms are to a large extent screened from the effects of 

 these rays by the earth's atmosphere, but this would not be the 

 case in interplanetary space. 



Even if we were able to prove that living organisms first 

 reached the earth from some other planet, however, it would 

 not help us in the least to understand how they first originated. 

 Such a hypothesis can only serve to remove the scene of action 

 from the earth to some unknown sphere where the investigation 

 of the problem is altogether beyond our reach. We may just 

 as well assume at once that the first terrestrial organisms were 

 generated in situ upon the earth itself and endeavour to find out 

 how such generation may have occurred. 



This brings us to our second alternative, which we may speak 

 of as the hypothesis of spontaneous generation, or, if we prefer 

 Huxley's term, abiogenesis. The discussion of this question has 

 unfortunately been greatly prejudiced by the hasty conclusions 

 of various observers who from time to time have announced that 

 they have actually witnessed the production of living organisms 

 from not-living matter, a claim which has been repeated at 

 intervals ever since people began to speculate on such subjects, 

 but which no one has yet succeeded in substantiating. I shall 

 refer presently to the latest efforts in this direction, but in the 

 meantime we must carefully bear in mind that the sudden 

 appearance of recognisable organisms where none previously 

 existed, and in situations to which no living things can have 

 gained access, is a very different thing from the gradual evolution 

 of living matter from inorganic substances by slow and imper- 

 ceptible steps, which are at first purely chemical and physical in 



