xxxviii FACTS AND OBSERVATIONS. 



ON THE DURATION OF SPECIES (chiefly). This is an important part of vital statistics, which, 

 running up the whole scale of existences, reaches and deeply interests man himself. This subject 

 introduces us to masses of time beyond our conception, but not beyond the vital force of some 

 genera and species to endure. Families and orders are little affected by the flux of time, or of con- 

 ditions, but genera more so some, however, living through many periods. We shall see that, with 

 certain exceptions, the life of a species is far shorter, while that of an individual is incomparably so. 

 So numerous are their generations, that it is idle to speak of them as less than hundreds of thousands. 

 Continually liable to unforeseen occurrences, the duration of Molluscaii communities is very 

 precarious ; it may be stopped by the loss of a genus. 



Oscillation is the parent of many of the direct causes interfering with organic existence of all 

 classes. 



As a general law, the viability of a species is in the inverse ratio of its organic rank each 

 species, nevertheless, having its own quantity of endurance. With individuals this law is reversed ; 

 the higher the rank the greater the viability. While the wants remain the same, the means of satis- 

 fying them are more effective. The points of contact with external things less expose them to damage, 

 than they are means of support and safety. The individual man lives longer than the monkey, the 

 horse than the fowl, the bird than the oyster. The duration of individual life may perhaps be mea- 

 sured by the rate of development (M. Dufo). This subject is yet in its infancy. 



We suppose the various stages, whether Primordial, Oslo, Caradoc, Wenlock, or Niagara, to 

 have occupied a deal of solar time, while individual life was short. It might have been, as at 

 present, a day, a season, a year, or a term of years periods very brief in comparison with a stage, a 

 subdivision. 



The simpler organization of the Protozoa and Diatomacea, but especially their extreme 

 fecundity, enables them to resist successfully all the agents of extinction. 



Passing by the several vague opinions on this subject previously put forth, I shall only mention 

 that of Prof. Bronn and Mr. S. P. Woodward (with the priority here I am not acquainted). They 

 have concluded that a species generally lives through one-third or one-half of the duration of the 

 set of beds in which it appears. Although this statement wants precision, it was an approach to 

 the truth. M. Bronn, after much consideration, tells us that only a quarter or a sixth of the fauna 

 of a stage has a duration equal to that of the "terrain" containing it (Prize Essay, p. 357), and in 

 further proof quotes an interesting table from Mr. Searles Wood's Monograph on the Crag fossils 

 (Palseontograph. Soc. 1848) . This Table, with others, proves that the species ordinarily thought to 

 represent a " terrain/' or a fauna, only continue through a part of it. Darwin (Origin of Species, 

 p. 293) says that insuperable difficulties prevent any just conclusions on this point. 



Whatever may be thought of the above paragraphs, M. Barrande has poured a flood of light 

 upon us. For Bohemia he has given us a true relative measurement of organic duration, and 

 therefore more or less applicable to all other countries. M. Barrande has supplied the great want 

 a careful assignment of fossils to their proper places in their one or more stage-subdivisions. He 

 subdivides all the fossiliferous strata of Central Bohemia into parts 1, 2, 3, 4, 5, according to 

 observed peculiarities : see pp. xxiv, xxv. 



M. Barrande shows that of 396 species of Bohemian Orthoceratites, thirty-two began and 

 ended their existence in the first subdivision (E. e. 1), thirty-eight more passing into E. e. 2, there 

 to perish, and that one hundred and ninety-six appeared in E. e. 2 exclusively, not a single Ortho- 

 ceratite reaching E. e. 3, a subdivision entirely destitute of them (and of Cyrtoceras}. Fauna F. f. 1 

 has only five typical species ; F. f. 2 has seventeen, G. g. 1 has twenty-three, while G. g. 2 has 

 three, and G. g. 3 thirteen, H. h. 1 having only two. All cease to exist with their respective 

 little group of beds. Some recurrents have been omitted, but not many. 



The genus Cyrtoceras of Bohemia exhibits the following remarkable evidence of the brief life 

 allotted to its species. In fauna E. e. 1 there are twenty-seven species appearing and disappearing 

 within that subdivision, save ten, which ascend into E. e. 2, but no further ; while in E. e. 2 alone 



