PRINCIPLE OF DIVERGENCE. 



377 



dray-horses, &c., and then goes on to explain how the same 

 principle applies to natural species. " From the simple cir- 

 cumstance that the more diversified the descendants from any- 

 one species become in structure, constitution, and habits, by 

 so much will they be better enabled to seize on many and 

 widely diversified places in the polity of nature, and so be 

 enabled to increase in numbers." 



The principle is exemplified by the fact that if on one 

 plot of ground a single variety of wheat be sown, and on to 

 another a mixture of varieties, in the latter case the produce 

 is greater. More individuals have been able to exist because 

 they were not all of the same variety. An organism becomes 

 more perfect and more fitted to survive when by division of 

 labour the different functions of life are performed by differ- 

 ent organs. In the same way a species becomes more efficient 

 and more able to survive when different sections of the species 

 become differentiated so as to fill different stations. 



In reading the Sketch of 1844, I have found it difficult to 

 recognise the absence of any definite statement of the prin- 

 ciple of divergence as a flaw in the Essay. Descent with 

 modification implies divergence, and we become so habituated 

 to a belief in descent, and therefore in divergence, that we do 

 not notice the absence of proof that divergence is in itself an 

 advantage. As shown in the Autobiography, my father in 

 1876 found it hardly credible that he should have overlooked 

 the problem and its solution. 



The following letter will be more in place here than its 

 chronological position, since it shows what was my father's 

 feeling as to the value of the Sketch at the time of its com- 

 pletion.] 



C. Darwin to Mrs. Darwin. 



Down, July 5, 1844. 



I have just finished my sketch of my species theory. If, 

 as I believe, my theory in time be accepted even by one com- 

 petent judge, it will be a considerable step in science. 



