450 ONE FAUNA OF MANY AGES. 



in the way of identifying strata by fossils, for he observes that if the 

 coasts on two sides of a sea are so situate that the one is subject to oscil- 

 lations, rising above or falling beneath the general ocean level, while 

 the other remained firm, we should obtain different results. In the 

 one case there would be the exuviae characteristic of a marine deposit 

 with unchanged mineralogical structure, while, in the other, there 

 might be great variety in the life, or a destruction of species which 

 continued to exist unchanged in the other situation, so that there 

 would be little or no resemblance in the organic contents of contem- 

 poraneous rocks. Similar views were long afterwards restated by Mr. 

 Herbert Spencer in an anonymous article entitled " Illogical Geology," 

 since reprinted. 1 



Subsequently, in 1862, Professor Huxley 2 discussed the question 

 in an address to the Geological Society, and after demonstrating that 

 geology can only prove a local order of succession, goes on to state that 

 for areas of moderate extent no practical inconvenience is likely to result 

 from assuming the corresponding strata to be synchronous. But the 

 moment the geologist has to deal with the large areas or completely 

 separated deposits, then the mischief of confounding that homotaxis, 

 or " similarity of arrangement," which can be demonstrated, with 

 synchrony or identity of date, for which there is not a shadow of 

 proof, under the one common term of contemporaneity, becomes incal- 

 culable, and proves the constant source of gratuitous speculations. 

 Hence 3 it is possible that similar or even identical faunas and floras, 

 in two different localities, may be of extremely different ages, if the 

 term age is used in its proper chronological sense. 



Barrande's Colonies. Edward Forbes enunciated the doctrine 

 that when a species is once extinct it never reappears. But the dis- 

 appearance of a species from a stratum succeeding that in which it 

 is found, does not, however, imply extinction. Barrande conceived 

 of the idea that assemblages of species, forming the fauna of a sea-bed, 

 might migrate away in one age, and return again to the same district 

 in a later period of time. This first appearance of the same fauna he 

 terms a colony. The Cambrian rocks of Bohemia exhibit many examples 

 of such colonies. The reality of these colonies has been attacked and 

 defended. The great difficulty in accepting them is not so much the 

 inherent improbability of the repetition of the same life group in the 

 same district, as the universal absence of such a phenomenon in the 

 newer strata. This assumed repetition reminds us of the old assertion 

 on which the name Lower Silurian was first instituted, that the fossils 

 found in the Upper Silurian also characterised the Lower Silurian. 

 This repetition existed, and is now well known to have been due to 

 inversion of the strata, so that the Lower Silurian were the Upper 

 Silurian. Mr. J. E. Marr has similarly suggested that the colonies of 

 Barrande are due to the same strata being repeated by inversion and 

 contortion. 



Persistent Types of Life. In the succession of life exhibited 



1 Essays. 2 Huxley Address, Geol. Soc. 1862. 



3 Huxley Address, Geol. Soc. 1870. 



