EVOLUTION 177 



Darwin's explanation, and the so-called discontinuous 

 variations of De Vries's explanation. The differences 

 of opinion have to do with the method of variation pro- 

 duction, that is, variation that may result in a new 

 species. 



After variation is secured, there is no question as to 

 the function of selection. It is merely a statement of 

 fact to say that some variations persist and some are 

 eliminated. It is a very different matter to claim that 

 only the "fit" persist. In some way the selection is 

 made, and the selection factors may be quite variable. 

 In general, it may be said that there is no serious differ- 

 ence of opinion that evolution is based on variation and 

 subsequent selection. It is only a matter of detail to 

 determine the exact factors. 



There is a much more serious problem of evolution, 

 however, which is still baffling. The variations ob- 

 served, which result in new species, as tested by genetics, 

 and for which the cytological machinery has been ob- 

 served, produce species either laterally or retrogres- 

 sively; that is, species of the same phylogenetic level, 

 or of declining rank. There is as yet no adequate ex- 

 planation of progressive evolution, the advance from 

 one great phylum to another. Progressive evolution is 

 a very evident fact, as shown by many an impressive 

 series disclosed by the geological records, and also by 

 our inferred lines of phylogeny. The theory of ortho- 

 genesis is often cited as an attempt to explain progres- 

 sive evolution. Orthogenesis is not an explanation, but 

 a statement of the fact of progressive evolution, which 

 still awaits explanation. The multiplication of species 



