386 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Non-living environment. 

 a. Passive. 



Isolation of areas ; continuity of areas ; building material ; 

 food ; place of concealment ; temperature ; humidity. 



aa. Active. 



Pressure of earth, water, and air. 



II. Living environment. 



Food ; reproductive potency ; sexual selection ; digestive 

 and other physiological power ; muscular strength ; su- 

 perior weapons and other special mechanisms ; intelli- 

 gence. 



I have already (page 4) quoted the language of 

 Darwin, where he states that the supposition that nat- 

 ural selection is a cause of the origin of variations is a 

 mistake. From the nature of selection, Darwin's po- 

 sition thus expressed, is self-evidently sound. The 

 attempt has, however, been made to apply the term 

 selection to the efficient cause of all variation, and to 

 divide its exhibitions into two kinds, natural and arti- 

 ficial selection. If the primary assumption involved 

 in this position is illogical, the dual division proposed 

 is absurd. As may be readily seen in the table in the 

 preceding paragraph, in which the factors of natural 

 selection are enumerated, the conditions necessary to 

 selection are mostly identical, whether imposed by na- 

 ture or by the hand of man ; i. e., whether natural or 

 artificial. The physiological effects of food, tempera- 

 ture, exercise, etc., do not differ, whether due to nat- 

 ural conditions, or to the influence of man. The ob- 

 servation of man's influence is indeed especially in- 

 structive in increasing our knowledge of the effects of 

 natural causes, since in the former case we have the 

 process in action within our control, while in the latter 

 case it is not. 



The subject of natural selection has been ably 



