THEORIES OF THE ORIGIN OF SPECIES 17 



increase of living things. From these two facts it 

 follows that when a change of environment takes 

 place, certain members of an existing species will be 

 somewhat better adapted than others to withstand 

 the new conditions, and the former will tend to survive 

 to the exclusion of the latter. It is assumed that 

 during a long series of generations this process will 

 cause a steady change in the character of the species in 

 the direction of better adaptation to the new conditions. 

 Thus we might suppose that among the ancestors of 

 the snakes those which happened to possess the longest 

 and thinnest bodies and the smallest limbs had the 

 advantage over their fellows that they were able to 

 crawl through narrower holes, and that for this reason 

 a greater number of them survived to produce off- 

 spring. Here we have a better basis for reasoning 

 than the supporters of Lamarck's doctrine, because 

 we actually know that longer parents, in whom this 

 quality was apparently not the result of taking thought * 

 do tend to produce on the average longer offspring. 



3. The view of the mutationists, already fore- 

 shadowed by Aristotle, and in recent years especially 

 associated with the names of Bateson and de Vries,; 

 expresses the conclusion that the evolution of new 

 species has taken place principally by the help of 

 variations of the discontinuous kind. By this process 

 there can arise at a single step new forms which have 

 already the complete and definite character usually 

 associated with a species specially adapted to particular 

 conditions. Of these new forms, those which happen 

 to be fitted for their surroundings as well as or better 



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