28 
R. Ruggles Gates. 
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 palaeobotanists 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. Each 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. 
