44 MAN: 



slight variation in habits and mode of life which the 

 physical forces of the universe are ever producing. 

 As external conditions are ever changing under the 

 operation of physical forces, and this in conformity to 

 ■established laws, so we may rest assured that variations 

 in life-forms are equally the orderly results of secondary 

 causation, though we may not in the present state of 

 knowledge be able to indicate either the time when 

 or the mode in which such causation may operate. 

 Every anatomist, every breeder of animals, and every 

 propagator of plants, knows that variations do take 

 place — every palaeontologist and fossil-collector per- 

 ceives that similar variations have taken place ; but 

 neither in the existing nor in the extinct has the 

 process been traced far enough, nor have sufficient 

 data been accumulated to enable science to determine 

 the full efficiency of this principle as a cause of specific 

 and generic distinctions. 



But though observation has not yet been enabled 

 to complete the argument, there can be no doubt of 

 the existence of the principle of variation, or of the 

 important part it plays in the modification of life- 

 forms ; and we may safely* accept it as one of the 

 main factors in the law of biological development. 

 Variation takes place so slowly, and by stages so 

 minute, that ages may pass before it rises into what 

 we are in the habit of calling " specific" distinctions ; 



