AGASSIZ. 87 



istence is limited within a definite period. Of genera 

 he says, " Genera are groups of animals most closely 

 connected together, and diverging from one another 

 neither in the form nor in the composition of their 

 structure, but simply in the ultimate structural peculiari- 

 ties of some of their parts." " Individuals, as representa- 

 tives of genera, have a definite and specific ultimate struc- 

 ture, identical with that of the representatives of other 

 species." 



We may pronounce these definitions to be mere 

 phrases, and inquire with Haeckel: " Of what nature 

 are these ' ultimate structural peculiarities of some of 

 their parts ' which are supposed alone to define the 

 genus as such, and to be exclusively characteristic of 

 each genus? We ask every systematizer whether he 

 may not equally well apply this definition to species, 

 varieties, &c., and whether it is not finally the ' ulti- 

 mate structural peculiarities of some of their parts ' 

 which produce the characteristic forms of the species, 

 the variety, &c.'' In vain do we search in the " Essay 

 on Classification " for a single example of the manner 

 in which, for instance, the genera of oxen or antelopes, 

 the races of hysenas and dogs, or the two great genera 

 of our fresh-water bivalve shells, the Unio and Ano- 

 donta, are actually distinguished by " the ultimate struc- 

 tural peculiarities of some of their parts." Several of 

 these definitions given by Agassiz may be interchanged 

 point-blank, so general and merely negative are their 

 statements. He characterizes the classes " by the man- 

 ner in which the plan of the type is executed as far as 

 ways and means are concerned." The orders, " by the 

 degree of complication of the structure of the types." 

 7 



