304 PALAEOZOIC TIME — DEVONIAN AGE. 



continue thus into the Carboniferous ; the genus Productus, whose 

 earliest species are very small and few, are afterwards of large size 

 and numerous. These changes do not consist in the gradual varia- 

 tion of the earlier species ; for species are essentially constant in 

 their characteristics. They become apparent solely in the succes- 

 sion of species, — the later species created differing in the particulars 

 mentioned from those which preceded them. 



Each of these points admits of extensive illustration ; but the 

 above is sufficient to give an idea of the kind of progress life was 

 undergoing. Each period had its new creations and its extinctions, 

 and often, also, there were many successive creations and extinctions 

 in a single period. Families and tribes were in constant change ; 

 and through all these changes the system of life was in course of 

 development. 



Threads running through past time to the present era increased 

 very slowly in number ; for to the genus Lingula, which, if correctly 

 determined, reaches to the farthest distinguishable limit in the his- 

 tory of animal life, with Nautilus, Rhynchonella, Pleurotomaria, Crania, 

 and Discina, of the Silurian, only Nucula, Terebratula, and perhaps 

 Area, are added in the course of the Devonian age. These genera 

 are all Molluscan. Every other (exclusive of some comprising the 

 inferior organisms called Rhizopods, see p. 163) becomes extinct 

 before the Age of Man. The early life was thus cast off, as the 

 earth became adapted to new forms in the expanding system. 



DISTURBANCES CLOSING THE DEVONIAN AGE. 



In eastern Canada, Nova Scotia, and Maine, the Devonian and 

 Silurian strata are uplifted at various angles beneath unconform- 

 able beds of the Carboniferous (Dawson, Logan, C. Hitchcock). 

 These uplifts preceded the Carboniferous age (or at least the Car- 

 boniferous period) ; but the precise epoch of their production — 

 that is, whether before the close of the Devonian age, or not, or 

 whether partly in the Silurian — is yet to be ascertained. Those 

 in Maine, according to Hitchcock, probably took place at the 

 close of the Devonian ; and the same may be true of the others 

 alluded to. 



In Iowa, Wisconsin, and Illinois, there are also similar uplifts; 

 but the Lower Carboniferous strata are involved in the disturbance, 

 and these, with the subjacent beds, lie unconformably beneath the 

 Coal formation. Here, therefore, the uplifts must have taken place 

 partly at least (perhaps wholly) after the period of the Lower Car- 

 boniferous. 



