4 Mutations and Evolution. 



Any future evolutionary principle which aspires to 

 universality must consist in a synthesis or integration of these 

 factors, or such of them as can he proved to he 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 

 J record are no longer applicable.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. 



V 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 

 V_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 



