114 THE EVOLUTION OF MAN. 



ancestors and allied forms. Moritz Wagner worked out this 

 point most excellently under the name of " The Theory of 

 Migration." 38 But, in our opinion, this famous traveller 

 has over-estimated the importance of his " Theory of Mi- 

 gration/' in so far as he declares it to be a condition 

 necessary to the rise of new species, and holds the " Theory 

 of Selection" to be incorrect. The two theories are, howr 

 ever, in no way opposed. On the contrary, migration, 

 by which the ancestral species of a new kind becomes 

 isolated, is only a special form of selection. The great and 

 interesting series of chorological phenomena, since they can 

 only be explained by the Theory of Descent, must also be 

 considered as important inductive data of the latter. 



Exactly the same is true of all the remarkable pheno- 

 mena which, in the " Household of Nature," we observe in 

 the economy of the organisms. All the various relations of 

 animals and plants, to one another and to the outer world, 

 with which the QEkology of organisms has to do, and espe- 

 cially such interesting phenomena as those of parasitism, of 

 family life, of the care of young, and of socialism, — all admit 

 of simple and natural explanation only by the Doctrine of 

 Adaptation and Heredity. While it was formerly usual to 

 marvel at the beneficent plans of an omniscient and bene- 

 volent Creator, exhibited especially in these phenomena, we 

 now find in them excellent support for the Theory of 

 Descent ; without which they are, in fact, incomprehensible. 



Finally, the whole of Ontogeny, the history of the indi- 

 vidual evolution of all organisms, is an important inductive 

 foundation of the Theory of Descent. But as this subject 

 will be especially treated in later chapters, nothing further 

 need be said of it here. Step by step, I shall endeavour 



