FACTS AND OBSEKVATIONS. xxxix 



there are 199 which are never seen elsewhere. The remaining thirty-two species, which make up 

 the 258 existing near Prague, are scattered about in faunas F. f. 1, 2, and G. g. 1, 3 each species 

 in one subdivision only. 



The Bohemian Brachiopoda are 321 in number as known at present. Out of this aggregate, 

 223 species (my stock until within a few weeks), only thirty- nine live in one horizon, or about one- 

 sixth ; but the determinations being sometimes imperfect, this statement is only partially reliable. 

 Ninety-eight more species from the same generous hand have reached me recently. They are in 

 fourteen genera. Of these only four species appear in more than one part of a subdivision. 



The Bohemian Trilobita are in 352 species ; respecting half of them there are good data. Of 

 this half (189 really), 127 live only in one part of a subdivision, thirty-four go into the next stage, 

 and twenty-eight into several parts of the same faunal subdivision. 



The Bohemian species of Pteropoda are ninety-seven in number. Seventy-seven do not out- 

 live their native subdivision, such as D. d. 5, E. e. 2, or F. f. 2, &c. 



This Prague area has 248 known species of Gasteropoda. Of these, 232 die out at the close 

 of their respective subdivisions, in whatever stage they may be. 



An examination of the other orders, Dimyaria &c., would only lead us into a repetition of the 

 above statements. We see that, leaving the recurrents out for the present, a species only exists in 

 Bohemia during a part of a stage-subdivision, and that the organic separation of part from part is 

 very sharp leaving but a brief interval for the exercise of natural selection. 



The fauna of the earth appears in these times to have been renewed incomparably more 

 frequently than has been supposed. In a Presidential Address by Prof. Ramsay (Quart. Journ. 

 Geol. Soc. vol. xx. p. 59) are many valuable statements on this subject. They were arranged and pre- 

 pared by E/. Etheridge, Esq., F.R.S.E., and, although directly applicable to the Oolitic formations, 

 agree in the main with the observations just made on the Silurian fauna of Bohemia. Every geo- 

 logist should make himself familiar with this address. 



We can only stop for the present to draw two important conclusions. 



The act of creation was continuous and frequent throughout every part of every stage of an 

 epoch, conditions permitting. 



Some species had a prodigious duration ; that of the greater number we have seen to be short 

 relatively. Measured by solar time, by the vast extent of molluscan dispersion, and by the rate of 

 sedimental accumulation in recent times, this duration was always great. 



EXTINCTION. All beings are made finite by the action of two laws. The great First Cause has 

 impressed upon all creatures a certain rate of progress (during youth), maturity, and decadence, 

 ending in extinction. Then, again, all beings are subject to external conditions, favourable and 

 unfavourable, which assist in the production of an average longevity. These may be called the laws 

 of impress and of conditions. Extinction of life is commonly slow, continuous, individual, and 

 sometimes is more rapid than replacement from without or than by acts of creation. Sudden acts 

 of extermination are exceptional, brief in time, and limited in space. 



The extinction of an order is veiy rare : it implies the lapse of more than one epoch or era. 

 In the same way the disappearance of a genus is of more importance than that of a species, because 

 it tells of a greater change of conditions. 



Local groups consisting of one species are not uncommon ; but the great bulk of animal life is 

 formed into societies more or less complex, their members living in a state of mutual dependence. 

 A number of their species, or even of their genera, may disappear without the dissolution of the 

 society, because compensations may arise from various sources. Examples of these societies are 

 numerous in the great work of Sedgwick and M'Coy (Benson Knot, Dudley, &c.), in the writings 

 of Mr. Billings (to whom I owe so much) , and others ; but they may be constructed in almost any 

 number, with singular exactitude, from the Silurian basin of Bohemia *. 



* Mr. Lycett (Journ. Geol. Soc. Lond. vol. iv. p. 42) gives an interesting example of a community from the Great 

 Oolite near Minchinhampton. 



