28 THE THEORY OF EVOLUTION 



convincing for that reason, because in science, as 

 in the practical affairs of life, probability must be 

 the guide, whenever certainty is not reached and 

 certainty is very rarely reached, if ever. 



Speaking of plants, Dr. D. H. Scott says: "Our 

 ideas of the course of descent must of necessity be 

 diagrammatic; the process, as it actually went on, 

 during ages of inconceivable duration, was doubtless 

 infinitely too complex for the mind to grasp, even 

 were the whole evidence lying open before us. We 

 see an illustration, on a small scale, of the complexity 

 of the problem, in the case of domesticated forms, 

 evolved under the influence of man. Though we 

 know that our cultivated plants, for instance, have 

 been developed from wild species within the human 

 period and often within quite recent years, yet noth- 

 ing is more difficult to trace, in any given instance, 

 than the true history of a field-crop or garden plant, 

 or even, in many cases, to fix its origin with cer- 

 tainty." 1 Under the infinitely more complex con- 

 ditions and longer time, in which natural evolution 

 is believed to have taken place, it is no reason for 

 surprise that the evidence should be not only in- 

 direct, but also general, even sometimes vague, rather 

 than specific in character. On the other hand, what 

 lends great cogency to this evidence is the fact that 

 several quite distinct and independent lines of proof 

 all converge in a common conclusion. While per- 

 haps no one of these lines is altogether complete or 



1 D. H. Scott: Studies in Fossil Botany, London, 1900, pp. 524-5. 



