1869.] The Treasures of jSiluria. 35 



must not tiierefore be confounded with the law which ascribes 

 the greatest range to the most lowly organized animals. 



The general law of the range of species in space and time may 

 therefore be broadly and roughly stated as follows : — long life 

 and great range ; short life and restricted range. Now, without 

 at all doubting the fact that the lives of species, like those of 

 individuals, may vary in length to a great extent, we think that 

 naturalists who " count heads " should satisfy themselves whether 

 a species which has spread over three-fourths of the globe, and 

 enjoyed an existence extending through several divisions of the 

 Silurian period, is precisely equivalent in Natural History value 

 to a species of the same genus which, with scores of others, was 

 both created and destroyed within the limits of one minor sub- 

 division of the same period, and which never extended beyond an 

 area of a few square miles. To put this question in a concrete 

 form, let us ask whether Orthoceras amiulatum is of an equi- 

 valent value to, say, 0. infermixtuni? The former is a species 

 which ranges from the Caradoc to the Ludlow rocks inclusive, and 

 from New York, through Northern Europe and Great Britain, to 

 Bohemia, while the latter occurs in only one subdivision (E.e.2) 

 of the Silurian system, and in but one small district in Bohemia. 



Taking into consideration the whole of the facts, illustrated as 

 they are by the few which we have described, we feel inclined to 

 believe that whereas some species had strong constitutions, and 

 could sustain changes in conditions and the wear and tear of travel 

 with comparative impunity, — in other words, that they possessed 

 great vitality (Dr. Bigsby calls it viability), — others jiresented 

 more feeble resistance to such untoward cu'cumstances. These 

 latter either died out altogether or became so far altered as to merit 

 new names at the hands of Pala3ontologists, — who unfortnately are 

 generally (and frequently of necessity) guided only by the laws of 

 convenience and the rule of thumb. 



If we are correct in our estimate of the relative value of species, 

 the very awkward question will arise, What inferences can legiti- 

 mately be drawn from Dr. Bigsby 's synoptical tables? In other 

 words, What is the use of "counting heads?" Echo answers, 

 What ? And we believe that we may strengthen our position with- 

 out violating the truth by ascribing this phrase "counting heads," 

 as well as the first denunciation of the practice, to an eminent geo- 

 logist who at one time evinced considerable skill in its exercise. 



We have now discussed at some length the first appearance and 

 the duration of species, and our space compels us to pass on to a 

 hasty review of the causes which have led to and accompanied their 

 exthiction. Dr. Bigsby's statements on this subject are not a httle 

 confusing, as some of them include the idea of the action of purely 

 local, and chiefly geographical causes, while others are equally dis- 



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