Introduction. 3 



the incidence of natural selection. Nevertheless this increased 

 precision is of vast importance, because it is based upon a multitude 

 of actual breeding experiments and accurate observations. 



But while mutationist conceptions have been coming in as a 

 refinement of Darwinism, other elements of the problem have not 

 been left without attention. Much has been written concerning 

 isolation, the adaptation of geographic races, and orthogenesis ; 

 while even crossing has been raised by some to the importance of 

 an evolutionary factor. While these pages are primarily a discus- 

 sion of mutation as an evolutionary factor, an incidental considera- 

 tion of some of the other factors will be necessary, in order to give 

 mutation its proper setting in the picture. 



As mentioned above it appears probable that the era of the 

 vain search for a single evolutionary principle is now at an end. 

 That end has been chiefly brought about by the experimental 

 researches in genetics on the one hand and the work of animal 

 palaeontologists on the other. While palseobotanists have been 

 actively accumulating valuable historical data, it does not appear 

 that palaeobotany has yet much direct evidence to offer concerning 

 the causes or methods of evolution in plants. The reason for this 

 probably lies in the greater difficulties of method involved in 

 palaeobotanical research. The animal palaeontologist may be able 

 to determine or identify an animal from the mere outline of a 

 single bone or a few portions of the skeleton. The fossil botanist 

 must rely largely upon sections of material which is so preserved 

 as to show the details of histological structure. 



All the known evolutionary factors have in turn had their 

 advocates, who usually attempted to make their application 

 universal. We have had the Lamarckian factor of use and disuse, 

 natural selection, isolation, direct adaptation (" epharmosis "), 

 orthogenesis, mutation, the unpacking of Mendelian factors, and 

 crossing, as explanatory principles. Their failure in universality 

 has been apparent enough in every case. On the other hand, 

 many of these factors and perhaps all, may be reasonably claimed 

 to have had some share in the evolutionary result. Bach advocate 

 may be expected to press his claim as far as possible, but there are 

 obvious limiting factors with regard to every one of these principles 

 the moment we begin to examine them ; and it is only by limiting 

 the field of vision or deliberately closing our eyes to other facts, 

 that a belief in the universal application of any one of them can be 

 retained. 



