10 EVOLUTION THE KEY TO NATURE 



tion. They believe that there are sometimes very 

 marked advances at a single birth (which they call 

 " mutations " = changes) , and it is these which are the 

 great agency of progress. So you get three chief types 

 of theory, and, passing over other shades of opinion 

 as unsuitable for this small work, we may express 

 them thus : 



1. Darwinism. Progress is due to the selection 

 by nature of the fitter to survive. This really means 

 the destruction by nature of the less fit to survive, 

 so that strength, weapons, senses, etc., are gradually 

 improved, just as we gradually improve our sheep 

 and cattle. Few now hold that this is as complete 

 an explanation as Darwin thought. 



2. Weismannism. That the action of nature is 

 secondary, and the advances of animals and plants 

 are determined in the germ (or "germ-plasm"). 

 Progress is gradual, by small improvements in each 

 generation. This theory of Professor Weismann is 

 not held as much as it was twenty years ago. 



3. Mutationism or Mendelism. Small changes or im- 

 provements in offspring would be lost or "swamped." 

 Large changes, in which the offspring differs consider- 

 ably from the parent, often occur, and these make 

 new species. This theory provides a very elaborate 

 scheme of the elements of heredity. It takes its name 

 from Abbot Mendel, but is chiefly due to Professor 

 Hugo de Vries. It has to meet many difficulties, and 

 is far from generally received. 



