93 THE AMERICAN NATURALIST [VOL. XL VII 



of adaptations: first, adaptations of form and function to 

 different kinds of environments ; and second, the natural 

 selection of the function of irritability, or, in other words, 

 to the selection of adaptability, or the adaptation to 

 changeableness of environment. Selection of the first 

 kind of adaptations may have given rise to varieties, 

 species, genera of the same type of animal, and have pro- 

 duced the spreading, or diversification ; while selection of 

 the second kind of adaptation may have produced the 

 movement onward and upward of all animal forms. 



These two kinds of adaptations do not always go 

 together and selection of the one may outweigh the other. 

 It is because selection to a specific environment some- 

 times is more important than selection of adaptations to 

 changeableness, that not all organisms have progressed 

 in the scale of evolution equally rapidly: but some have 

 persisted in special environments with slight changes of 

 structure for very long periods, or may even have retro- 

 gressed; while other forms, in which the second adapta- 

 tion has been rigorously selected, have moved rapidly on- 

 ward and upward, and show little adaptation to any 

 special environment. 



The question whether evolutionary progress is due to 

 the selection of this second adaptation, that of adapt- 

 ability, occurs very naturally to a physiologist, because, 

 in the first place, the evolutionary development of con- 

 sciousness and intelligence appears to him to be one of 

 the most important, if not the most characteristic move- 

 ment in evolution ; and in the second place, his point of 

 view in considering evolution and adaptation is some- 

 what different from that of the zoologist or the paleontolo- 

 gist. To him the organism does not appear constituted 

 of bones, skin, horns, or other structures, but to be consti- 

 tuted essentially of a number of mechanisms in activity, 

 each mechanism having a definite function to perform. 

 Evolution, for the physiologist, is not evolution of struc- 

 ture primarily, but evolution of function; and he natu- 



