36 EVOLUTION 



tion, then, decidedly weakly specimens, if they occurred 

 in nature, would have practically no chance of sur- 

 vival, and would consequently never be seen. This 

 attempt to undermine one of the strongest evidences 

 of organic evolution, therefore, falls to the ground. 



7. THE OBSERVED FACTS OF MUTATION. As a 

 matter of fact, novel types are seen in nature not in- 

 frequently, and are specially common in some groups 

 of plants, as Mr. C. T. Druery has shown for the case 

 of the British ferns ; and isolated specimens of par- 

 ticular wild species belonging to other families are fre- 

 quently found, which, if they had occurred as con- 

 stant features of a considerable group of individuals, 

 would afford a basis for the description of a new species. 

 The study of mutation will, however, require a special 

 chapter of its own. 



We see, thereforej that the evidence in favour of the 

 existing species of animals and plants, having arisen 

 by a process of evolution, is of a most ample and con- 

 vincing kind. The theory of organic evolution is, 

 however, incomplete until we have arrived at a true 

 account of the method or methods by which new 

 species arise from old ones. The earliest definite 

 explanation, as already stated, was that given by 

 Lamarck, and we may next proceed to consider the 

 Lamarckian theory of the origin of species. 



Earlier writers had already supposed that species 

 became modified through the action of the external 

 conditions to which they were exposed. Lamarck laid 

 special stress upon the observed facts that the organs 

 of individuals become increased and developed through 



