10 OEGANIC EVOLUTION — PHYSICAL 



liave been due to evolution only, or, as the single other 

 alternative, to an immense series of special creations, 

 made not only when life began, but also during its 

 entire continuance, in exact imitation of a process of 

 evolution — a preposterous theory, for which there is no 

 more warrant than for the hypothesis that our various 

 breeds of dogs have not descended from a common 

 ancestry, but were separately created. 



The third great mass of evidence is that furnished 

 by experiment. It is therefore the most conclusive of 

 all, since any theory which can be proved experimentally 

 may be said to be proved decisively. For instance, if 

 we have a theory that the boiling-point of water varies 

 with the atmospheric pressure, and find on experiment 

 that it does so vary, we may conclude that we have 

 proved our theory beyond cavil. As a disputed point 

 is involved, which it will be advantageous to discuss 

 thus early, we will consider this part of the evidence 

 more at length than we did the rest. The theory of 

 evolution may be put to the test of experiment. We 

 may take any species of plant and animal, and by 

 copying the process of natural selection, by destroying 

 those individuals of the species which we regard as 

 unfit, — i.e. those which vary unfavourably as regards 

 any peculiarity we wish to develop, — and by allowing 

 only those individuals who vary favourably as regards 

 that peculiarity to continue the race, we may develop 

 the chosen peculiarity in the chosen species to almost 

 any extent; the one condition being, that our operations 

 shall extend over a sufficient number of generations. 

 The efforts of man to improve his cultivated plants and 

 domesticateid animals may be described as a lengthened 

 series of such experiments, conducted on a gigantic scale, 

 with the result that he has been able to vary plant and 

 animal structures almost at will, and so greatly that, in 

 the absence of scientific and historic testimony, no one 



