126 CALIFORNIA ACADEMY OF SCIENCES. 



evolution. Natural selection implies a contest between 

 a more and less favorable class of variations, in which 

 the less favorable class perish. As an example, a race 

 horse might be evolved by artifical selection from a 

 common stock, but in order to produce a dray horse 

 from the same stock, the two varieties would have to be 

 isolated from the earliest stages of their divergence. 



The tendency of this view of Mr. Gulick's is to over- 

 estimate the self-sufficiency of the organism to originate 

 new forms by virtue of an assumed inherent tendency 

 to vary indefinitely. In other words, he sometimes 

 appears to assume, in conformity with Nageli, that each 

 organism contains within itself the potentiality of devel- 

 oping all the forms which may be subsequently derived 

 therefrom; that free intercrossing suppresses this ten- 

 dency to vary, but isolation enables each group to 

 develop any little idiosyncrasies toward which it may 

 have a leaning, and by successive stages of isolation 

 these are piled up into new characteristics or structures. 

 Thus, in summing up the result of his survey of the 

 various forms of selection, he finds: *" First, that all 

 the forms of Reflexive Selection are due to the relations 

 of members of the same species to each other, and are 

 liable to change without any change in the environ- 

 ments. Second, that Active Natural Selection is due to 

 change in the successful use of the powers of the organ- 

 ism in dealing with the environment, and is not depend- 

 ent on change in the environment. Third, that Passive 

 Natural Selection, which is due to the exposure of the 

 organism to a different environment, is often produced 

 by the organism's entering a new environment without 

 there being any change in either the new or the old 

 environment. Fourth, that when Passive Natural Selec- 



* Journ. Linn. Soc. xxiii, p. 337. 



