ENVIRONMENT AND EVOLUTION OF VERTEBRATES 31 



vanced but it has lost its power of adaptation to changing 

 environment and is therefore not generalized. 



We wonder in what manner changing environment has in- 

 fluenced the evolution of the vast number of mammalian types 

 which exist or have existed on the face of the earth. The most 

 closely related species are not found living together ; they will 

 always be separated by some barrier of mountain or water. In 

 species formation it is usual to consider at least four aspects, — 

 variation, heredity, selection and segregation* of which the 

 last has already been touched upon. Regarding variation it is 

 necessary to define the term more precisely. Those variations 

 which are heritable are known as mutations. Obviously there 

 can be three types of mutation, those without effect upon the 

 relation of the animal to its environment, those which produce 

 a type less in harmony with its environment, and those which 

 render it in closer harmony. Of these the persistence of the 

 first is accidental; animals exhibiting the second must ulti- 

 mately become extinct by the action of natural selection ; those 

 in which the third type of mutation occurs become dominant 

 and persistent. Segregation permits of the independent evolu- 

 tion of mutations which we shall eventually find occur linked 

 in vast numbers appearing always in certain well defined direc- 

 tions through the inherent tendencies of the germ-plasm. We 

 are concerned in this book with those mutations only which are 

 of importance in the relation between the animal and its en- 

 vironment. 



We are indebted to Waagen, an Austrian paleontologist, for 



*It must be conceded that these aspects differ greatly in complexity and the mean- 

 ing of all except the fourth varies according to the concept of the writer. Of them 

 segregation is simply one environmental factor ; selection is not creative, but in Os- 

 born's metaphor a sieve for the results of interaction of environment (animate and 

 inanimate surroundings), ontogeny (laws and forces operating in the development of 

 the individual) and heredity (laws and forces operating in the germ-plasm) ; variation 

 is the expression partly of environment, partly of ontogeny as just defined, partly of 

 heredity. Further there is interaction of which we are as yet utterly ignorant between 

 the forces of heredity on the one hand and those of environment and ontogeny on the 

 other. The initiation of new characters is not the special perquisite of any one of 

 the three factors environment, ontogeny, heredity, but is common to all. Thus upon 

 closer examination it is seen that the four terms used in the text, though apparently 

 simple enough, are really the expression of considerable confusion of thought. The un- 

 known agency which works through heredity and is termed by Bergson the original 

 impetus, we shall discuss in Chapter XXI. For further information along these lines, 

 consult Osborn: Tetraplasy, Jour. Acad. Nat. Sc, Phila., 1912, series 2, xv, 273 



