No. 610] 2IUTATI0N THEORY AND SPECIES-CONCEPT 581 



denly, at one step, was considered practically equivalent 

 to its creation by a miracle, and the type of argument in- 

 volving this view is still not infrequently leveled against 

 the mutation theory. 



But where lies the necessity for assuming that either 

 continuity or discontinuity is universal? Surely the 

 matter is one to be determined by direct observation, and 

 not by a priori argument. Tlie continuity concept of 

 origins appears not to have influenced other sciences to 

 the extent it has biology. True, Lyell first introduced it 

 by developing the doctrine of uniformitarianism in geol- 

 ogy. Nevertheless the geologist has continued to deal 

 with large and relatively catastrophic effects occurring 

 at irregular intervals, such as landslides, floods, earth- 

 quakes and volcanic eruptions. The phenomena of ge- 

 ological history are then continuous only in a limited 



Similarly, no chemist supposes it necessary to think 

 that, for example, carbon and silicon were gradually dif- 

 ferentiated from some previous substance which pos- 

 sessed certain qualities of both. On the contrary he sees 

 his atoms built of definite units, the electrons, combined 

 in various ways and numbers to give a variety of prod- 

 ucts, the elements, which are for the most part stable 

 from the tirst. Hence, while perhaps little can be gained 

 for the biologist by reasoning from analogy with other 

 sciences, yet we at least realize that concepts of discon- 

 tinuity are quite as widespread in science at large as are 

 those of continuity, and that the origin of a character is 

 not explained merely by breaking it up into infinitesimal 

 steps through which it may not have passed at all. 



Let us consider now some concrete instances. And here 

 we shall select chiefly cases of discontinuity, since we are 

 considering especially the bearing of the mutation theory 

 on the conception of sjiecies. In examining s]iecies and 

 genera of plants and animals, we find very often, par- 

 ticularly in plants, charactoi s wlii('li alnu^st certainly had 

 a discontinuous origin. Perliaps the majority of generic 



