318 



GENERAL ZOOLOGY 



shift in a plus direction (Fig. 182). Thus, through the partial or 

 complete elimination of minus individuals in each generation, 

 the variation of Graph 1, representing the original distribution 

 of the character, would shift in a plus direction to position 2, 

 position 2, then to 3, and finally to 4, which represents a variation 

 range entirely beyond the upper limit of the original. The 

 character would have undergone a complete change as a result 

 of the natural selection of the more favorably endowed individ- 

 uals in each generation. 



If the same argument held for the sum total of the variable 

 parts, there would result in later generations an entirely different 

 sort of animal from the original one. According to Darwin, 



2 3 



Fig. 182. — The hypothetical effect of selection on a single character. 



the origin of a new species results from such a gradual accumula- 

 tion of slight variations under the guidance of natural selection. 



On the other hand, discontinuous variations, provided they are 

 favorable, would hasten the process of change, since a discon- 

 tinuous variation represents a greater difference from the mode 

 than the most extreme fluctuating variation. Darwin under- 

 stood this but refused to attach much importance to discon- 

 tinuous variations as a basis for species change, because he 

 believed they occurred too infrequently to be of selective value. 



Darwin assumed that all variations are inherited, which 

 of course simplified his explanation of the action of natural 

 selection. It is now known that many fluctuating or continuous 

 variations are not inherited and while this fact does not destroy 

 Darwin's theory, it limits the action of natural selection to the 

 role of an eliminating agent rather than that of a creative factor 

 in the evolution of new species — natural selection determines 

 whether or not a character shall survive after it has appeared; 

 but the causes leading to its appearance are independent of 



