Natural History of the United States, 257 



It only remains on this subject to remark that the practical 

 difficulty of the discrimination of species occurs only in excep- 

 tional cases. When we endeavour by external characters, such 

 as proportions of parts, external ornamentation, &c, accurately to 

 distinguish forms of the same origin, we may, it is true, be deceived 

 in some rare cases by the similarity of really distinct species, or 

 the variations of the individuals of the same species. But, when 

 we consider the well-defined limits of form, ornament, &c, in the 

 greater number of animals, we cannot doubt that accurate atten- 

 tion to all the facts bearing on these will enable us eventually to 

 solve the most intricate cases, without having recourse to any 

 hypothesis destructive of the true unity of the species. 



"We have aimed in the above remarks only to show that grave 

 difficulties beset the view of species advocated by Agassiz, and 

 that such views, if carried to their legitimate results, would des- 

 troy all certainty in Zoology, quite as effectually as that opposite 

 view which would so enlarge the limit of specific unity as to ad- 

 mit that any number of species may have descended from a com- 

 mon parentage,. 



As might have been expected, a mind so familiar with nature 

 as that of Agassiz clings to the truth on this practical view of the 

 subject, however far from it in the mere theory of species. Hence 

 the able reasoning in this work on the immutability of species, 

 their range of distribution in time and space, and the care neces- 

 sary in their discrimination and description. On these last sub- 

 jects the following paragraphs are well worthy of attention, though 

 some of the considerations referred to are vastly more important 

 than others : — 



"If we would not exclude from the characteristics of species 

 any feature which is essential to it, nor force into it any one which 

 is not so, we must first acknowledge that it is one of the charac- 

 ters of species to belong to a given period in the history of our 

 globe, and to hold definite relations to the physical conditions 

 then prevailing, and to animals and plants then existing. These 

 relations are manifold, and are exhibited : 1st. in the geographiea. 

 range natural to any species, as well as in its capability of being 

 acclimated in countries where it is not primitively found ; 2d. in 

 the connection in which they stand to the elements around them, 

 when they inhabit either the water, or the land, deep seas, brooks, 

 rivers and lakes, shoals, flat, sandy, muddy, or rocky coasts, lime- 



