190 Nature and Origin of the 



defined limits. This proposition may be illustrated by a figure, 

 in which species are represented by circles, many of which are 

 in contact, and whose areas are sprinkled irregularly with dots, 

 which represent varieties. One central dot represents the type 

 of the species. Some larger dots represent types of a value 

 intermediate between that of species and ordinary varieties. 

 The amount of difference between the types may be repre- 

 sented by their distances. On the boundaries of the species 

 we find varieties which closely resemble their neighbors in the 

 adjacent species, while their affinities with the central types 

 of both species are so nearly balanced, that it is not really a 

 matter of much consequence on which side of them the ima- 

 ginary boundary line of the species is drawn. On a plane 

 surface, however, we have only an approximation to the truth. 

 A more exact representation of the relations of the types would 

 require the three dimensions of space. It should be observed 

 also, that the boundaries of the circles do not represent any 

 facts, which have an objective existence. AVith the boundary 

 lines, we represent the species as described in books ; without 

 them, we see the species as they exist in nature. 



If all the examples of this kind should be enumerated, 

 very few species would remain isolated. Of such, some might 

 be united by further discoveries ; while others might remain 

 isolated, since it is a part of the general plan of organic nature 

 that the spaces between the groups shall be unequal, so that 

 some species, some genera, some families, &c., shall stand 

 quite alone. 



The principle is not peculiar to the terrestrial mollusca of 

 Jamaica, The Naiadae and the Melanidae of the United 

 States, which have been so thoroughly studied by Mr. Lea, of 

 Philadelphia, are exactly in the same case, but the facts are 

 expanded over wider geographical limits. The same is true 

 of the snails and fresh water shells of Europe, of many groups 

 of marine mollusca, of fishes, of birds, and even of mammals. 



