192 SUMMARY OF CONCLUSIONS [ch. xv 



12. Varieties are the last stages in the mutation, and are not, 

 as a rule, incipient species. 



13. Chromosome alterations are probably largely responsible 

 for the mutations that go on. 



14. The theory of natural selection is no longer getting us any- 

 where, except in politics (influence of the dead hand). 



15. It comes in principally as an agent to fit into their places 

 in the local economy of the place where they are trying to grow, 

 the forms there furnished to it, whether newly evolved, or only 

 newly arrived, killing out those in any way unsuitable. 



16. It has, therefore, not been responsible for the progress that 

 has been made by the actual evolution of new forms, but it has 

 been all-important in fitting them into their places in the economy, 

 which is always increasing in complexity. 



17. The theory of natural selection makes evolution a con- 

 tinuous and gradual process, diff'erentiation a discontinuous one. 



18. Natural selection (the struggle for existence) works rather 

 upon individuals than on groups. It causes the survival of the 

 fittest population, rather than the fittest type in the mixture. 



19. It can make few or no predictions, while diff'erentiation, 

 like age and area, can make many, which are usually successful. 



20. Adaptation has been mainly internal or functional, rather 

 than external or structural. 



21. Differences in structure do not necessarily mean difiPerences 

 in adaptation. 



22. The mutations supposed in diff'erentiation would at one 

 step cross the "sterility line" between species, which has always 

 been a great stumbling block to natural selection; and thus at 

 once isolate the new form, preventing its loss by crossing. 



23. Diff'erentiation makes it possible for evolution to go on 

 more rapidly than under natural selection. 



24. It explains the great discontinuity seen in the facts of 

 economic botany. 



25. It explains the difficulty, almost insuperable to the theory 

 of natural selection, of the increasing divergences of characters 

 as one goes up the scale from species to family. 



26. It gets over the difficulty of early stages, and of the fre- 

 quent correlation of characters, and the need of calling in "mor- 

 phological necessity"; it does not need to call in adaptation, as 

 the theory of natural selection has to do ; and it explains why the 

 large genera are the most variable. 



27. It explains the fact that adaptation is so often generic. 



