239* LAKE SUPERIOR. 



analogues only by the most careful examination. This parallelism 

 is sometimes most exact, running not merely through the genera, 

 but even through the respective species of which they are composed ; 

 thus of the two species of Olisthgerus, each is most closely related 

 to its European analogue, 0. laticeps being similar to 0. megace- 

 phalus, while 0. nitidus can scarcely be known from 0. substriatus. 



While upon this subject, we may take occasion to distinguish the 

 different kinds of replacement of species, which are observed in pass- 

 ing from one zoological district to another more or less distant. 

 There appear to be four distinct modifications by which faunas are 

 characterized. 



1st. When the same species, or organic forms, so similar as to pre- 

 sent no appreciable difference, appear at points so situated as to 

 preclude the possibility of any intercommunication. These are most 

 rare, and are only observed when the physical circumstances under 

 which the species exists are nearly identical. 



2cl. When a species in one district is paralleled by another in a 

 different region so closely allied that upon a superficial glance they 

 would be regarded as the same. These are called analogous species ; 

 e. g., the Olisthreri, Spondyli, Bembidia, Helophori, &c., &c., of 

 the preceding catalogue, as compared with European species. 



3d. Where several species in one region are represented by 

 several others of the same genus, which perform a similar part in 

 the economy of nature, without, however, displaying any farther 

 affinity to each other. These are called equivalent species ; e. g., 

 most of the species of Cicindela, Brachinus, Clytus, Donacia, &c.,of 

 America, as compared with those of the eastern world. 



4th. Where the members of a group are represented collectively 

 by kindred species in another district, which however display such 

 differences of structure that each may at once be referred to its pro- 

 per locality ; e. g.,most of the Melolonthae among Coleoptera, and 

 the entire group of Quadrumana among mammalia. 



Now it will be observed, that in proceeding from the Arctic circle 

 to the tropics, the prominent character of the fauna is successively 

 modified by these peculiarities. We pass from a region where the 

 fauna is the same at remote points, through one where the produc- 

 tions are similar, but not identical, to one finally, where the equilib- 



