AGASSIZ. 87 
existence 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 pecu- 
liarities of some of their parts.” “Individuals, as repre- 
sentatives of genera, have a definite and specific ultimate 
structure, 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 hyzenas 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.” 
5 
