GEOGRAPHICAL DISTRIBUTION. IIC> 



Second, organisms now living are so distributed in relation 

 to the conditions of environment that we are led to recognize 

 this general law : that the morphological characters of organ- 

 isms are in some way associated with or related to the phys- 

 ical environment in which they live. 



Ancestry and Environment as Causes of Evolution. Thus, by 

 looking only at the superficial relation of organisms, i.e. 

 those which may be expressed in number and ratio, we find 

 a definite relationship existing between organic form (mor- 

 phology) and both geological time and physical conditions on 

 the earth's surface. We may express the relationship by the 

 proposition, that the morphological characters of any particu- 

 lar organism Jiave come to be what they are through the opera- 

 tion of two sets of conditions : first, the organic conditions- 

 which were antecedent to the appearance of the given organism; 

 and, secondly, the external physical conditions into which it was 

 born. The first set of conditions is expressed by the general 

 term Ancestry, and the second by the term Environment. 



Differences of Opinion respecting Interpretations not Facts. 

 So long as we confine our attention to the simple relationship 

 existing between organic structure and the passage of time or 

 the varying conditions of environment, we have touched only 

 the fundamental facts of the real problem before us. 



The series of correlated phenomena are as they are, what- 

 ever be our interpretation of them. The reason for first care- 

 fully spreading out the facts themselves is in order to show 

 that they are not invented by any theory, that they exist 

 independently of any preconceived view, and that the differ- 

 ences in opinions regarding them are not matters of observa- 

 tion, but are matters of philosophy. 



Introduction of Causation into the Discussion. And here we 

 introduce a new element into the discussion. We assume 

 that cause and effect are involved in their relationship. We 

 assume that in the course of time the organisms which went 

 before must bear the relation of determining cause to those 

 that follow, and that in physical space or environment, the 

 conditions of geographical locality are a determining cause in 

 relation to the species adjusted to particular natural-history- 

 provinces. 



