724 



THE AMERICAN NATURALIST [Vol. XLVII 



stinct has attained a high degree of precision and invari- 

 ability. In the same way, a person whose activities are of 

 wide range and comparative simplicity is much more 

 adaptable than one who has become habit-bound through 

 a life of intense specialization. As an organism's "ex- 

 perience mass ' ' becomes continually greater and more com- 

 plex the formation occurs of that system of habits which 

 in man we call a mental character, and this process, like 

 that of ontogenetic and phylogenetic development, in- 

 volves the continual elimination of potentialities and con- 

 sists in the progressive fixation, with advancing age, of 

 characters which during youth were variable and incon- 

 stant. It so much resembles the establishment of an 

 organic structural type by the elimination of variability 

 through advance in specialization as to suggest that per- 

 haps both phenomena are manifestations of the same 



Such attempted explanations of the differences in fixity 

 which occur between organic characters are of course in- 

 complete and highly unsatisfactory. The very fact, how- 

 ever, that it is possible at all to formulate principles of 

 conservatism and variability, unexplained though they 

 may be, which shall be of application throughout the 

 animal and plant kingdoms or which shall at least be 

 operative in certain definite groups of organisms, is of 

 great significance and value to the biologist, for it enables 

 him to place all branches of his science on a somewhat 

 more exact and uniform basis. It must of course be borne 

 in mind that such principles as these are not invariably 

 operative, for exceptions to all -of them are frequently 

 found. Biological laws undoubtedly exist, but they seem 

 to belong to quite a different category from the invariable 

 ones of the physical sciences. 



The science of taxonomy will perhaps receive the great- 

 est benefit from a general recognition of the fact that 

 there are such things as laws of phylogeny, for a united 

 effort by all biologists to define these laws more clearly 

 and to apply them more widely will result, through the 



