244 Transactions. 



used by him. It would be inadvisable to include the Brachiopoda in any 

 New Zealand comparisons, because it so often happens that no brachiopods 

 are to be found in the beds that contain molluscan fossils. 



A further objection to this method is based on the possibility of the 

 migration of faunas to New Zealand at different times. Our knowledge 

 of the fauna is probably sufficient now to enable us to express a definite 

 opinion on this subject, and it can be stated with some confidence that we 

 have every reason for thinking that at no time has an introduction of a 

 considerable number of species to our Tertiary molluscan fauna taken place. 

 It is certainly a fact that a statement of the number of Recent species in 

 the different fossil faunas will more speedily discover whether there has 

 been such a migration than any other method of research. 



It is now generally admitted that there is no stratigraphical break 

 in the series of Tertiary rocks, and also that there is none in the faunal 

 succession that is represented in them, for, as I have previously cjuoted. 

 Thomson and Morgan state " Each Tertiary fauna seems to merge gradually 

 into the succeeding one "* ; and the late Mr. Suter has written the following 

 in a letter to me : " There is no doubt that our molluscan fauna has 

 gradually decreased, and also that the Tertiary forms gradually merge 

 into one another." If this is the case it will certainly be extremely hard, 

 if not impossible, even when our Tertiary fauna is completely known and 

 the range of species definitely recorded, to find a horizon at which any 

 important number of species disappear simultaneously. It is distinctly 

 better under these circumstances to characterize the beds as containing 

 certain percentages of Recent species. 



This statement of the ])roportion of Recent species will, however, not 

 provide a secure basis for comparison with European horizons. The small 

 size of New Zealand and the fact that the more recent researches and the 

 number of autochthonous species in its fauna and flora all show that the 

 land has been isolated from other countries since, at the latest, the late 

 Cretaceous, except possibly in the post-Tertiary, suggest that the organic 

 changes must have been relatively slow. This belief is further supported 

 by the knowledge that there has been no great variation of climatic con- 

 ditions : that is, though the climate has been warmer and colder, it has not 

 been tropical at any time, nor has there been anything that has approached 

 the arctic cold of the Great Ice Age of Europe. 



Again, it must be emphasized that our strata were deposited under the 

 most different conditions of depth of water and of proximity to the shore- 

 line, so that it is almost certain that in different parts of the area con- 

 glomerates, greensands, and limestones were contemporaneous formations. 

 Such different lithological horizons would necessarily have also very different 

 palaeontological facies, and a very great deal of work must be undertaken 

 before we can be reasonably certain of their contemporaneity if we rely 

 upon the occurrence or absence of index fossils. On the other hand, the 

 Recent fauna contains species that live under all these different conditions, 

 and it therefore affords a basis for comparison with the fauna of every 

 lithological facies of the Tertiary strata. 



It follows that if there is in New Zealand a continuous succession of our 

 Tertiarv molluscan species and an unbroken series of Cainozoic strata, and 

 if the contemporaneous strata have a great variety of lithological facies, 



* Preface to Palaeontological Bulletin No. 5, 1917. 



