Il8 GEOLOGICAL BIOLOGY. 



of the organisms, as expressed in their geological sequence, 

 the order of the phenomena appears like a mere unfolding of 

 successive phases of organic life upon the globe, each phase 

 preparing the way for the next ; and had we no preconcep- 

 tions, I think this evolution would seem to be the most 

 natural thing in the world. 



Gradual modification with each step of generation would 

 be found in each case the sufficient explanation and cause of 

 that which followed. 



But when it is observed that each living organism is 

 closely adjusted to a particular set of environmental condi- 

 tions, and that specific organic form and specific conditions 

 are closely co-ordinate factors, the question as to the influence 

 exerted by environment upon the organism becomes a prob- 

 lem of equal importance. 



An Explanation required for Succession of Species as well as for 

 Adjustment of Species. — The study of the relations of organisms 

 to geological time and to geographical space first brings out 

 the simple fact that differentiation of organic form is actually 

 related to both. There is an adjustment of the organism to 

 each of the phenomena, time-succession and place-extension. 

 If we turn from this simple statement of fact to seek for some 

 reason why organisms differ in form, and why one organism 

 has one form and another organism of another time and place 

 differs from it, then there appears back of geological succes- 

 sion and of geographical distribution an element of causation. 

 There are conditions in the succession and in the distribution 

 which we may suppose have been the cause, or at least the 

 occasion, of the changes of form exhibited by the organism. 



Evolution and Adaptation both observed Facts. — We have 

 already remarked that the examination of a series of forms in 

 the rocks shows the modification and change in their form to 

 be co-ordinate with progress of time, and on following them 

 from the lowest rocks upward through the geological column 

 to the present, each series ends in recognized living organisms ; 

 hence we conclude that it is a characteristic of organisms to 

 pass through continuous change in time. This process of 

 changing morphological characters, expressed in the history 

 of organisms, is called Evolution. 



