44 EVOLUTION, SOCIAL AND ORGANIC 



of a large white Doris (a kind of sea slug) 

 how extraordinarily numerous they were. 

 From two to five eggs (each three thousandths 

 of an inch in diameter) were contained in a 

 spherical little case. These were arranged two 

 deep in transverse rows forming a ribbon. The 

 ribbon adhered to the rock in an oval sphere. 

 One which I found, measured nearly twenty 

 inches in length and half inch in breadth. By 

 counting how many balls were contained in 

 a tenth of an inch in the row, and how many 

 rows in an equal length of the ribbon, on the 

 most moderate computation there were six 

 hundred thousand eggs. Yet this Doris was 

 certainly not very common: although I was 

 often searching under the stones I saw only 

 seven individuals. No fallacy is more common 

 among naturalists, than that the numbers of 

 an individual species depend on its powers of 

 propagation.'' 



This instance is moderate compared with 

 multitudes of others. The question then arises 

 as to why, of such a numerous progeny, only 

 a sufficient number reach adult stage as will 

 replace the parent stock so that population 

 remains practically stationary. 



Here Darwin became indebted to Dr. 

 Malthus who, but for that indebtedness would 

 have been forgotten ere this. In his "Essay 



