No. 554] ADAPTATION AND THE PHYSIOLOGIST 92 



to understand on the basis of Darwin's explanations. 

 Among these difficulties, most of which were recognized 

 by Darwin, there are the phenomena of parallel evolu- 

 tion among different species and genera, which, though 

 diverse, appear all to be moving forward in the same 

 direction; the phenomena of steady, limited progress in 

 one direction which point toward orthogenetic variation ; 

 the phenomena of the appearance of rudiments and their 

 development until useful. It is exactly these difficulties 

 upon which physiology throws some light; and it is of 

 them that I particularly wish to speak. 



In the evolution of animals two movements may be 

 perceived : a spreading out and a progress ; a diversifica- 

 tion and a movement forward. The first movement is 

 illustrated by the formation of many different species 

 in one genus; or of many genera of the same type of 

 animal ; the second by the movement forward in the line 

 of evolution of all these species. These two movements 

 were not sharply distinguished by Darwin, but they have 

 been more or less clearly recognized by several philoso- 

 phers. It is this double movement which has given the 

 animal kingdom the form of a branching tree instead of 

 a single trunk. Darwin dealt mainly with the first of 

 these movements, which gives rise to genera, species and 

 varieties; which is shown by the diversification of ani- 

 mals and plants in domestication by human selection; 

 and he explained it by the progressively better adaptation 

 of forms to particular environments. He believed the 

 second movement, the movement upward, was due to the 

 same cause. 



It is the second movement which has been so hard to 

 explain and which has particularly puzzled the paleon- 

 tologist; the successive series of dominating types on 

 the earth's surface culminating in man; the progress 

 steadily toward the goal of consciousness and intelligence. 



The question which I wish to raise is whether these two 

 movements, which are at right angles to each other, may 

 not be due to the natural selection of two different kinds 



