36 EVOLUTION 
tion, then, decidedly weakly specimens, if they occurred 
in nature, would have practically ~o 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. 
4. THE OBSERVED Facts or 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, therefore, 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 
