108 DOCTRINE OF EVOLUTION 



now being formed with the strata of deeper levels; 

 these are so much alike that he is led to regard the 

 constructive influences of the past as identical with 

 those he can now watch at work. Similarly the biologist 

 must first learn, as we have done, the principles of 

 animal construction and development, and of other 

 classes of zoological facts, and then he must turn his 

 attention from the dead object of laboratory analysis 

 to the workings of organic machines. The way an 

 organism lives its life in dynamic relations to the 

 varied conditions of existence, as well as the mutual 

 physiological relations of the manifold parts of a single 

 organism, reveal certain definite natural forces at work. 

 Therefore his next task is to compare the results ac- 

 complished by these factors in the brief time they may 

 be seen in operation with the products of the whole pro- 

 cess of organic evolution, to learn, like the geologist in 

 his sphere, that the present-day natural forces are able 

 to do what reason says they have done in the past. 

 When the subject of inquiry was the reality of 

 evolution, it was perhaps surprising to find that even 

 the most familiar animals like cats and frogs provided 

 adequate data for science to use in formulating its 

 principles. So it is with the matter of method; it 

 is unnecessary to go beyond the observations of a day 

 or a week of human life to find forces at work, as real 

 and vital as animal existence and organic life them- 

 selves. This is true, because evolution is true, and be- 

 cause the lives of all creatures follow one consistent law. 

 Our task is therefore much more simple than most 

 people suppose it to be; let us look about us and 

 classify what we may observe, increasing our knowl- 



