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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