::0 ESSENTIAL CHARACTERISTICS OF SPECIES. 9 1 



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 relations of the parts, the orna- 

 mentation, &c. As in the absence of birds'-nests, snail- 

 shells, butterflies, &c., it is impossible, when it comes to 

 the erection of species, to pre-determine 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 Palaeontologists 

 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 



