FACTS AND OBSEKVATIONS. 



xlv 



when they are found in two different stages/' These statements favour the present inclination to 

 obliterate all sharp lines of demarcation. Beautiful examples abound in Wales and New York of 

 these transitions, by minute and prolonged shadings, mineral and organic. We have a striking 

 instance of this in Pennsylvania, in the passage of the Devonian series into Coal-measures 

 (H. D. Rogers). Recurrence usually deals with life, but it may be effected after death, by means 

 of transport, without our knowing it. When organic transition is sharp and sudden, between con- 

 formable strata, denudation may be suspected, as we see in the Black-River limestone of the Mohawk 

 valley (New York) ; but when it is seen in discordant rocks, there has been a positive break. 



Species are the principal time-tests ; genera and families run through so many stages and epochs 

 that they characterize none. Thus Lingulidae, Craniadse, Asteridae, Aviculidse, Nautilidse, &c. have 

 dwelt in the beds of almost every age (S. P. Woodward). In this point of view recurrents are often 

 of little value ; but they may be, nevertheless, essential to the well-being of a community. Thus a 

 ravenous Gasteropod in the course of his upward range, or a legion of them, may be sent in to 

 prevent the herbivorous class from exhausting the public store of food. 



The Table (X) now subjoined, presents a synoptical view of Silurian life in relation to the 

 subject in hand, as far as was known in 1865. It shows that out of 5968 species, whose places are 

 well known, 784 are recurrents, or 13 per cent. This leaves 5184 species faithful to one horizon. 

 Primordial recurrency has been left to another occasion ; it grows in importance. 



TABLE X. A Synoptical View of Silurian Life in reference to Vertical Range or Recurrency 

 known in 1865). The Primordial stage is treated apart. 



as 



This Table may be looked upon in several important aspects. The typical species of different 

 kinds are placed in succession, numerically, in their proper stages, together with their various 



m 



