Genus, Species, Varieties 63 



light or to sound or to touch or to chemical changes, and 

 even to forms of energy and to processes which he is him- 

 self unable to detect through his unaided senses. None of 

 these mechanisms, however, shows any special fitness to pre- 

 serve itself or to maintain itself. Moreover, none of these 

 artificial contrivances resembles a living thing in being able 

 to reproduce itself. So far as we know, all living things 

 come from preexisting living things. Where we know nothing 

 at all about the parentage of a plant or animal we assume 

 nevertheless that the individual originated exactly as did 

 other individuals of the same kind in the past. This fact of 

 generation has been universally observed wherever the origin 

 of a new individual plant or animal could be observed at all. 

 We therefore assume it to be true of all living things today, 

 even where it is impossible to observe directly the origin of 

 each particular individual. We further assume that this suc- 

 cession of individuals from generation to generation through 

 the processes of reproduction will continue indefinitely into 

 the future. Finally, we project this general fact backward 

 in time and assume it to have been true of all living things 

 in the past, even if we do not know when, or where, or how 

 the series started in the first place. Here again we have an 

 example of taking facts which are known to us in only a 

 limited number of instances and drawing from them the 

 feeling of certainty in regard to the past and in regard to con- 

 temporary events about which it is absolutely impossible to 

 prove anything. 



Genus, Species, Varieties 



We have assumed (page 51) that a number of beings 

 are ** related " in proportion to their resemblance. Accord- 

 ingly, we should say that all living things are related to one 

 another in so far as they do resemble one another, that is, in so 

 far as they are organisms with all the characteristics of living 

 things. This resemblance does not, of course, ** prove " that 

 they are indeed related or that they have, in fact, had com- 



