Organic Evolution 13 



It is but a short step in our thinking to ask whether the 

 world as a whole is changing. Has the world always been 

 the same as it is now, except for the ebb and flow of the 

 tides, for example, or except for some other reversing change? 

 Have the plants and animals that live upon the earth, and in 

 the air and in the water, always been the same in kind? Are 

 changes in living things only such as come with the changing 

 seasons, or have there been in the past forms of life different 

 from those we see today? In the absence of means for di- 

 rectly observing the facts on the earth before we were born, 

 one guess is perhaps as good as another. It is common sense, 

 however, to make use of whatever existing facts we can 

 gather, as well as of any other knowledge we can get as to 

 how things do happen — that is, as to the uniformities in 

 change, or the laws that appear to us to be enduring. 



Organic Evolution 



Things change. The materials of the universe endure. 

 Changes take place according to certain uniformities: noth- 

 ing happens without cause. Changes produce effects. The 

 workings of the world are the same in all parts; they 

 are the same always. It is common sense to be guided b}^ 

 facts and by certain kinds of inferences from facts. It is 

 common sense to assume that the workings of the world 

 which we can observe today are the same in kind as those of 

 past time. 



The main propositions regarding the evolution of life 

 upon the earth, then, are: 



(i) The animals and plants of today are descendants 

 of animals and plants that lived before them, and these in 

 turn were the offspring of other living things; 



(2) In the course of time the descendants of some 

 forms or types came to be different from their ancestors. 



There is continuity, but there is also change, or, briefly, 

 life presents a history of descent with modifications. 



While nobody has lived long enough to have observed 



