pilation of facts bearing on the question of evolution, and 

 by the masterly arguments for the origin of species through 

 the agency of Natural Selection. According to Darwin and 

 Wallace (co-discoverers of the Theory of Natural Selection) 

 there are three main factors in the evolution of species; 



(1) Over-production among organic beings; 



(2) Variation; and 



(3) Heredity. 



The main argument may be summarized as follows : 

 As organic beings tend to increase in a geometrical ratio, 

 while the means of subsistence tend to increasej)nly in an 

 arithmetical ratio (Malthus),i there naturally"^ises a se- 

 vere struggle for life at some age, season or year. As there 

 is much variation in the structure of even the most closely 



Fig. 2.— CHARLES DARWIN 



allied forms those forms that possess useful variations will 

 most likely survive in the struggle for existence^ in an 

 environment of infinite complexity, both with regard to 

 the relations of beings to one another and to the conditions 

 of life: then because of the "strong principle of inheritance 



(1)— Malthus' essay on "Population" was published in 1798 and the 

 conclusions were based on data relating to human population 

 obtained from many countries. 



(2)— Darwin said: "I use this term ('struggle for existence') in a 

 large and metaphorical sense, including the dependence of one 

 being on another, and including (which is more important) not 

 only the life of the individual, but success in leaving progeny." 



14 



