Mutations and Evolution. 
29 
Any future evolutionary principle which aspires to 
universality must consist in a synthesis or integration of these 
factors, or such of them as can be proved to be sound in the fields 
of their application. The time for unlimited advocacy of single 
exclusive factors is past. Future advance in the understanding of 
evolution must then consist in the determination of the limitations 
of each factor, and the weaving together of such elements as are 
sound, into a connected whole. 
The recent attempt of Osborn (1918) to deal with organic 
evolution from the energy standpoint, while not as yet markedly 
successful, may perhaps represent a line of approach through which 
a synthesis of evolutionary factors, may ultimately be reached. 
Whitman’s (1919) elaborate studies of evolution in pigeons are 
grounded upon energy conceptions. The great accumulation of 
palaeontological material which has taken place in recent years, 
gives vertebrate palaeontologists a solid substratum of fact on which 
to construct hypotheses, which experimentalists can no longer 
afford to ignore. Gaps in mammalian phylogenies, for example, 
have been filled up to a remarkable extent, so that many of the 
older arguments drawn from the imperfection of the geological 
record are no longer applicable. The wonder is rather at the 
completeness, variety, and abundance of the skeletons preserved 
in deposits and unearthed by man. 
A re-reading of the Origin of Species —an exercise which 
any biologist could profitably indulge in, at least once in every five 
years—serves on the one hand to emphasize the greatness of 
Darwin’s vision, and on the other to encourage the present scientific 
man by showing the immense accumulation of accurate knowledge 
concerning organisms which has taken place since Darwin’s time. 
The problem of to-day, just as it was in 1859, is to bring all these 
facts to bear upon the explanation of the diversity of organic 
species. In connection with this task not one of the modern 
biological sciences can be safely ignored, and such physical sciences 
as astronomy, geo-morphology, physiography, chemistry and many 
aspects of physics must be pressed into service. Darwin dealt 
with variation in wild and domesticated animals and plants; 
heredity; the relations of organisms to each other and to their 
inorganic environment, hybridization and sterility; the geological 
record ; geographical distribution ; and embryology ; in so far as the 
knowledge of his time permitted. In every one of these fields, 
except perhaps the broader principles of plant distribution, the 
