THE INDIVIDUAL AND THE SPECIES 187 



ancestors. Now the individuals composing such an 

 assemblage would exhibit a close resemblance to 

 each other, such a resemblance as our categories of 

 naturally occurring organisms are seen to exhibit. 

 We should also find that the individuals of our natur- 

 ally occurring assemblage would be able to interbreed 

 among themselves, just as in the case of the experi- 

 mentally produced population. It may be concluded, 

 then, that the naturally occurring population is also 

 the product of a pair of ancestors. This inter-fertility, 

 as well as the close morphological resemblance of the 

 individuals, are the facts on which the hypothesis of 

 the common origin and unity of the assemblage, or 

 species, is formed. 



The morphological resemblance between the 

 individuals, either in the natural or the artificial 

 populations, is not absolute. If we take any single 

 character capable of measurement we shall find that 

 it is variable from organism to organism. This 

 important concept of organic variability may be 

 made more clear by a concrete example. Exam- 

 ination of a large number of cockle shells taken 

 from the same restricted part of the sea-shore, 

 and therefore belonging presumably to the same 

 race, will show that the number of the radiating 

 ridges on the shell varies from 19 to 27, and that 

 the ratio of the length to the depth of the shell 

 also varies from i : 0.59 to i : 0.85. In the former 

 case the most common number of ridges is 23, and 

 in the latter case the most common ratio of length 

 to depth is i : 0.71. These are the characteristic 

 or modal values of the morphological characters in 

 question, and the other or less commonly occurring 

 values are distributed symmetrically on either side 

 of the mean or modal value, forming " frequency 



