8YNCHKONI8M OF GEOLOGICAL FOEMATIONS. 231 



or contemporaneous. The singular uniformity in the lithological 

 character of many of the equivalent formations also favours this con- 

 clusion. It is true that a very limited number of cases are known, or 

 at least have been cited, -where an old-type fauna is found interca- 

 lated vrith. a fauna of newer date, reversing, as it were, the general 

 order of succession or deposition. Such are the " colonies " which 

 M. Barrande has indicated as existing in the Silurian basin of Bo- 

 hemia, and which have been so forcibly dwelt upon by that investi- 

 gator as standing in opposition to any slow-modification theory of 

 descent and progression. Of a similar character are the colonies to 

 which attention has been called by Marcou; and the occurrence, 

 which has been observed in some parts of Scotland, of an Upper Si- 

 lurian fish-fauna in rocks of unquestionably Devonian age, may be 

 placed in the same category. It appears practically certain, however, 

 that in none of these cases is there a true reversion. In the Siluro- 

 Devonian faunal association of Scotland we have, doubtless, only one 

 of those common occurrences where the range of certain species of 

 animals has to be extended beyond what was supposed to be its far- 

 thest limit ; for, as Mr. Geikie informs us, there can be no question 

 that the Silurian element was present in the seas immediately ad- 

 joining the Devonian basin during the period of the first deposition 

 of the Devonian rocks, and that it was subsequently admitted into 

 the last by the rupture of a separating barrier. The case of the Bo- 

 hemian colonies is, perhaps, not as readily explained; but, if they 

 exist at all, it is very likely that they represent a fraction of a fauna 

 which, through specially favourable conditions, has been permitted 

 to linger beyond the period marking the extinction of the fauna as a 

 whole, and which, through a short migration, has been transported 

 to its present quarters. It must be confessed, however, that our 

 knowledge respecting these colonies is very imperfect, and it is a 

 significant fact that their existence, in the sense which has been 

 preferred by their interpreter, M. Barrande, is completely denied 

 by Lapworth, Marr, and others. 



In a recent address, delivered before the British Association for 

 the Advancement of Science (1884), Mr. Blanford has brought out 

 numerous facts tending to prove, in the language of the author, 

 " homotoxial perversity," or the want of synchronic correspond- 

 ence existing between certain identical, or closely related, assem- 

 blages of fossil remains. It is shown, for example, that the famous 



