NO ESSENTIAL CHARACTERISTICS OF SPECIES, Q1 
utter discord as to the limitations of species prevailed, 
and still prevails, that no agreement can be arrived at 
respecting the basis of the description of species, the 
“essential characteristics.” Although Agassiz lays down 
the diagnosis of the species, a decision is required in each 
case as to the mutual rclations of the parts, the orna- 
mentation, &c. Asin the absence of birds’-nests, snail- 
shells, butterflies, &c., it is impossible, when it comes to 
the erection of species, to pre-dctermine what may be 
the “essential characteristics” of the species they are to 
form, subjective opinions and arbitrary decisions have 
full play; and within a certain domain, well known by its 
forms, there are among the systematizers no two autho- 
rities who are agreed as to the number of species into 
which the material before them should be divided. 
The most unbridled license in the manufacture of 
species prevailed, however, among the Palontologists 
during a period when, in the endeavour to fix the sub- 
divisions of geological strata as accurately as possible by 
means of their organic contents, the separation of species 
was carried incredibly far, into the most minute and often 
into individual deviations. A certain mutability of species 
could not fail to obtrude itself on the most purblind eye; 
ramifications were made of sub-species, sports of nature, 
and varieties characterized by “less essential” peculiari- 
ties acquired by means of climate and inheritance. 
There was, however, always a reservation that their 
crosses with one another and with the main species 
should produce fertile offspring, whereas towards other 
species their relations were identical with those of the 
main species. Of course, in this separation of the 
species into sub-species, subjective opinion was even 
