354 THE president's address. 



that life can originate de novo, but in every case the assertion, 

 when critically examined, has proved to be baseless. On the 

 other hand, the experience of many hundreds, even thousands, 

 of investigators, working daily in bacteriological and other labora- 

 tories, affords a vast amount of accumulated support for the 

 following statement : When a suitable culture-medium, natural 

 or artificial, which has been previously sterilised, is planted, 

 under proper precautions, with a particular species of organism, 

 that species, and no other, develops in the culture. There is, in 

 fact, such an enormous mass of evidence favouring the beUef that 

 an individual organism of any kind whatsoever is the offspring 

 of a parent or parents similar in kind to itself, that the burden 

 of proof rests on those who put forward statements to the contrary. 

 Until, therefore, it has been clearly proved in a single instance 

 that new life can come into being without having arisen from 

 pre-existing life, we are fully justified in assuming that it 

 cannot do so. 



Proposition II. There must have been a period when life did 

 not exist on this earth. 



This proposition follows inevitably from a considei'ation, first, 

 of what is known positively with regard to living bodies ; secondly, 

 of the deductions of astronomers and others with regard to the 

 past history of our planet. In the first place, all living things 

 at least, all those known to us are extremely sensitive in regard 

 to even moderately high temperatures. Some bacterial spores can 

 survive being boiled in water for a short time, or require a tempera- 

 ture slightly above the boiling-point of water to kill them ; but 

 most living beings succumb at a temperature much lower than this, 

 and there is certainly no form of life known to us which would 

 not be destroyed instantly at a low red heat. Yet astronomers 

 assure us that there was a time when our terrestrial globe was 

 incandescent, and it seems as certain as anything can be that no 

 life could have existed on the earth at that epoch. 



Taking these two propositions together, that life does not now 

 originate de novo, and that there was an epoch in which it could 

 not have existed on the earth, it follows that there must have 

 been some period of past time in which the teeming life of our 

 planet first made its appearance probably in some simple and 

 primitive form that has given rise by a process of gradual evolu- 

 tion and differentiation to the immense variety of form and 



