EVOLUTION AS A NATURAL PROCT-SS ] I 1 



of its parents or like any one of its earlier ancestors ; 

 while furthermore in no case does an individual re- 

 semble perfectly another of its own generation or 

 family. This departure from the parental condition, 

 and the lack of agreement with others even of its 

 closest blood-relatives, are two familiar forms of 

 variation. As a rule, the degree to which a given 

 organism is said to vary in a given character is most 

 conveniently measured by the difference between its 

 actual condition and the general average of its species, 

 even though there is no such thing as a specimen of 

 average nature in all of its qualities. In brief, then, 

 variation means the existence of some dilTerences 

 between an individual and its parents, its fraternity, 

 and, in a wider sense, all others of its species. 



Passing now to the causes of variation, all of the 

 countless deviations of living things can be referred 

 to three kinds of primary factors ; namely, the environ- 

 mental, functional, and congenital influen('(\s that work 

 upon the organism in different ways and at different 

 times during its life. We shall learn that the evolu- 

 tionary values of these three classes are iiy no means 

 equal, but we take a long step forward wIumi we realize 

 that among the things we see every day are facts 

 demonstrating the reality of three kinds of natural 

 powers quite able to change the characters of organic 

 mechanisms. 



The ''environment" of an organism is everything 

 outside the creature itself. In tlie case of an animal 

 it therefore includes other members of its own kind, 

 and other organisms which prey upon its species or 

 which serve it as food, as well as the wliole -enr^s of 



