366 GEOLOGICAL BIOLOGY. 



decrease, and extinction of the type they represent, is of the 

 highest value as evidence of the actual order of evolution 

 and of the general laws by which differentiation of form has 

 taken place. And a few such cases far outweigh any num- 

 ber of detached specimens tied together by theoretical links. 



Natural Selection seems Eeasonable when Based alone upon 

 the Study of Living Organisms. When we observe living 

 animals in competition the vigorous ones living and the 

 weaker dying, the strong overcoming and devouring the 

 weak, the large and fewer in number making their daily 

 food of the smaller and more abundantly produced ; when we 

 note how the places for the greatest abundance of individuals 

 are determined by the presence of favorable conditions for 

 obtaining food ; and thus, in general, when we observe that 

 animals as they are are actually adjusted, each to its own 

 most favorable conditions of environment it seems reason- 

 able to draw the conclusion that the differences distinguishing 

 one animal from another may have arisen as the result of 

 better fitness for the struggle for existence on the part of 

 those which survived and carried on the race. 



Having once assumed that the law of evolution is a proc- 

 ess in which the chief active determining force has been nat- 

 ural selection by the survival of the fittest, it was easy to find 

 illustrations of adjustment of structure and function to the 

 conditions of environment among fossil, as has been done 

 among living, organisms. 



Every Species of Organism that has Flourished in the Past the 

 Fittest for its Place and Generation. When, however, we stop 

 one moment to consider the relations of organisms in the past 

 to their own environment, it becomes evident that, at every 

 particular stage in the geological history of organisms, the in- 

 dividuals then existing must have been as thoroughly well 

 adapted to live under the conditions of their environment as 

 the present inhabitants are adapted to live in their environ- 

 ment. Every organism that has lived on the earth has in 

 some sense been the fittest to live in the particular time and 

 conditions it occupied. 



If environmental conditions (outside of organic environ- 

 ment) have determined the evolution of organisms, then we 



