Chap. II. SPECIES. 16.3 



terizes genera? Is it not the finish of the organization of the body, as worked 

 out in the nUiinate details of structure, -wliich distinguishes one genus from another? 

 LatreiUe, in expressing the want he felt with reference to tlie study of genera, 

 has given us the key-note of tlieir harmonious relations to one another. Genera 

 are most closely allied groups of animals, differing neither in form, nor in com- 

 plication of structure, but simply in the ultimate structural peculiarities of some 

 of their parts; and this is, I Ijelieve, the best definition which can be given of 

 genera. They are not characterized by modifications of the features of the fami- 

 lies, for we have seen that the prominent trait of family difference is to be found 

 in a typical form ; and genera of the same finiily may not differ at all in form. Nor 

 are genera mei'ely a more comprehensive mould than the species, embracing a wide 

 range of characteristics ; for species in a natural genus should not present any 

 structural differences, but only such as exjiress the most sjiecial relations of their 

 representatives to the surrounding world and to each other. Genera, in one word, 

 are natural groups of a peculiar kind, and their special distinction rests upon the 

 ultunate details of their structure. 



SECTION VI. 



SPECIES. 



It is generally believed that nothing is easier than to determine species, and 

 that of all the degrees of relationship which animals exhibit, that which consti- 

 tutes specific identity is the most clearly defined. An unfailing criterion of S2)ecific 

 identity is even supposed to exist in the sexual connection which so naturally 

 brings together the individuals of the same species in the function of reproduc- 

 tion. But I hold that this is a complete fallacy, or at least a pctitio princijni, not 

 admissible in a philosophical discussion of what truly constitutes the characteristic^ 

 of species. I am even satisfied that some of the most perplexing problems involved 

 in the consideration of the natural limits of species would have been solved long 

 ago, had it not been so generally urged that the ability and natural disposition 

 of individuals to connect themselves in fertile sexual intercourse was of itself 

 sufficient evidence of their specific identity. AVithout alluding to the fact that every 

 new case of hybridity ^ is an ever-returning protest against such an assertion, and 



* WiEGMAN, Gekronte Prt'issclirift iiber die Bas- ton, (S. G.,) Essay on Ilybridil}-, Amor. Jour., 



tfirderzcugung im Pflanzenreich, Braunschweig, 1828, 1847. — Additional Observations on Hybridity in 



8vo. — Braun, (A..) Uebor die Erscbcinunn; dor Ver- Animals and on some collateral subjects, Charleston 



jiingung in dcr Natur, Freiburg, 184'.), 4to. — Mok- Med. Journ., 18.')0. 



