The Lifi'-Historji of Sinrifer hen's. 157 



of iiidividuals, living in one clatjfj of conditions, in suoii circuin- 

 stances as seem to warrant our calling tliem one physiological 

 species in the sense of l^eing able fertilely to cross Avith each 

 other, this being the explanation of the gradation of one form 

 into the other noted b}' Davidson. This presumed — that we 

 had a single species to begin with — we have, by intercrossing 

 and by local conditions modifying the offspring, well-defined 

 groups, which would be called races if we knew their Iiistory, 

 but which are called species because they appear at so widely 

 divided geological periods. 



These separate groups, however, decelop no new characters, 

 but in those ai)pearing at each stage are seen fixed and apparent 

 only varietal characters of the original form, with such modifi- 

 cations as poor, or rich, or varied food may give to animals we 

 now may modify. There is nothing of a specific character evolv- 

 ed in tliis series of forms which did not appear in the first forms, 

 but there is every evidence for the belief that the species has 

 lived through this long geological time without losing its char- 

 acter, and that all that has resulted from great time and change 

 of conditions has been the fixation into race-groups of the origi- 

 nal variable characters of the species. 



The species, at its first appearance in the Silurian, presented 

 a decidedly new combination of characters for the genus, and 

 also much variation. When once these specific though variable 

 forms appeared, they lived till the variations which could be 

 played upon them were exhausted; and the species ceased to live 

 and became extinct either near the close of the Carboniferous or 

 not till later in the Mesozoic. 



Some of the races or varieties may die out, but they reappear 

 again and again till there are such strong contrasts that it is 

 difficult to see even generic resemblance between them. 



The variety or so-called species of the Niagara Period, which 

 seems most closely to correspond to the form of S. Icevis, is 

 that mentioned in Vanuxem's Rejaort of the 3d District of New 

 York, p. 91, and called by Conrad Orthis hicostata, but not de- 

 scribed by him (see note in Pal. N. Y., Vol. 2, p. 263). This 

 was evidently a local variety, as Hall fails to discover it at any 

 considerable distance from the original locality; and what is re- 

 markable is the similaritv of conditions, as seen in the isolation^ 



