406 Transactions. 



an assumption to consider that the successive Tertiary faunas are entirely 

 direct descendants the one of the other, and that the curve obtained by 

 plotting .the percentages of Recent species against even divisions of time 

 is necessarily a straight line. If we suppose a large immigration of foreign 

 species at any one stage, and suppose further that the immigrants sub- 

 sequently became extinct or evolved into new forms in a greater proportion 

 than the endemic forms derived from the previous stage, then it is not 

 beyond the bounds of possibility that the later stage would show a lower 

 percentage of Recent species than the earlier one, although the actual 

 number of the Recent forms could not, of course, be smaller unless they had 

 temporarily emigrated elsewhere. A numerical example worked out in Ap- 

 pendix II to this paper will make this possibility more clear. Although 

 such a reversal of the increase of the percentage of Recent species in succes- 

 sively younger stages is very unlikely in the New Zealand area, still the 

 possible effect of immigration or emigration in making the curve irregular 

 must always be looked for. Until the course of the curve is much better 

 known it seems premature to base divisions of the time scale upon arbitrarily 

 chosen percentages of Recent species. 



It is, of course, true that the original classification of the European 

 Tertiary by Lyell was nominally based upon the principle of the percentage 

 of Recent species. There were special circumstances in this case which 

 caused the departure from the more ordinary methods, such as gave rise 

 to the divisions of the Jurassic or Cretaceous in England. Lyell was faced 

 with the problem of devising a classification for widely separate sets of 

 beds, the order of age of which could not be fixed by superposition. In 

 New Zealand we have not this difficulty". Lyell had in his mind, however, 

 and definitely mentioned, certain actual beds which would serve as types 

 for his different divisions — viz., the deposits of the London and Paris 

 Basins for the Eocene, the faluns of Touraine for the Miocene, the Sub- 

 apennine beds of Italy for the Older Pliocene, and still younger beds in 

 Sicily for the Newer Pliocene. The percentages of Recent species whicli 

 characterized his different divisions were not chosen or stated arbitrarily, 

 but were directly derived from the known faunas of these " type " beds. 

 When an increase in the knowledge of the fossils of these beds necessitated 

 a change in the value of the percentages, this change was accepted without 

 question by him, even although, as he stated, it rendered the derivations 

 of his names somewhat inaccurate. 



Stratigraphical classification resembles biological classification to this 

 extent: that the ultimate court of appeal must be not the idea set up by 

 the systematist in founding his species or division, but the actual and im- 

 mutable thing or type that lies behind it. A classification by percentages of 

 Recent species is based upon an idea liable to modification. A classification 

 by beds is based on things which for human purposes are immutable. 

 Behind any division of geological time based on a succession of faunas there 

 must be the actual beds which contain those faunas, and from which the. 

 nature of the faunas can be ascertained. There is no past fauna of which 

 our knowledge is complete, and it is not necessary to wait for a complete 

 knowledge before using the faunas for classification, provided the recks 

 containing them can be defined. This definition Marshall has neglected to 

 carry out, except so far as the names he suggests imply certain beds. 



If the principle of the type in stratigraphical classification is admitted 

 it may be desirable to import into stratigraphy the procedure used in zoo- 

 logical classification — viz., the right of a subsequent author to fix a type 

 when the original author has left it vague. It would hardly be fair at 



