Natural History of the United States. 255 



One farther remaik is rendered necessary by the illustration 

 above referred to. No one knows better tban Agassiz that to 

 compare, in reference to their geographical distribution, animals 

 nearly related, may often lead to errors greater than those likely to 

 result from the comparison of creatures widely different in struc- 

 ture but adapted for somewhat similar external conditions of ex- 

 istence. It is a fact very curious in itself, independently of this 

 application, that we find closely related species differing remark- 

 ably in this respect; and that, on the other hand, animals of very 

 different grades and structures are equally remarkable for wide 

 geographical ranges. The causes of these differences are often 

 easily found in structural, physiological, or psychical peculiarities, 

 but in many cases they depend on minute differences not easily 

 appreciable, or on the effects of geological changes. 



Fourthly. — Our author commences his dissertation on species 

 by taunting those who maintain the natural limits set to hybridity 

 with a petitio principii. The accusation might be turned against 

 himself. The facts shewing that species in their natural state do 

 not intermix, and that hybrids are only in exceptional cases fer- 

 tile, so enormously preponderate over the few cases of fertile 

 hybridity, that the latter may be regarded as the sort of exception 

 which proves the rule. The practical value of this character in 

 ascertaining the distinctions of species in difficult cases is quite 

 another question, as is the precise nature of the resemblances in dis- 

 tinct species which most favour hybridity, and the greater or less 

 fixity of the barrier in the case of species inhabiting widely sepa- 

 rated geographical areas, when these are artificially brought toge- 

 ther. Nor is the specific unity to be broken down by arguments 

 derived from the difficulty of discriminating or of identifying spe- 

 cies. The limits of variability differ for every species, and must 

 be ascertained by patient investigation of large numbers of speci- 

 mens, before we can confidently assert the boundaries in some 

 widely distributed and variable species ; but in the greater num- 

 ber this is not difficult, and in all may be ascertained by patient 

 inquiry. 



Fifthly, — The above considerations, in connection with the doc- 

 trines of created protoplasts, and the immutability of species, as 

 so ably argued by Agassiz himself, we hold irresistibly compel us 

 to the conclusion of Cuvier, that a species consists of the "beings 

 descended the one from the other or from common parents"; or 

 at least to that of De Candolle, that the individuals of a species 

 must " bear to each other so close a resemblance as to allow of 



