188 GEOLOGICAL DISTKIBUTIOIT. 



Among the Invertebrata we have also, as has already been inti- 

 mated, well-marked instances of apparent reappearance. Mr. Da- 

 vidson, in his review of the British fossil Brachiopoda," affirms 

 that the resemblance between the recent Rhynchonella nigricans, 

 which has nowhere as yet been found in any Tertiary deposit, 

 " and some Cretaceous and Jurassic forms is so great that we are 

 at a loss to define their differences." And, further, that certain 

 varieties of the Mediterranean Terebratulina caput-serpentis, whose 

 range extends downwards only to the Pliocene, so closely re- 

 semble the Cretaceous Terebratula striata as to render the two 

 barely, if at all, distinguishable. It may be that at some future 

 date intermediate links will be discovered to unite the forms of 

 the two periods; but who can, in the meantime, affirm that these 

 are not true cases of reappearance ? If it is true, as M. Barrande 

 asserts, that the Triassic Nautilidse show less affinity to existing 

 species than do the primitive forms — in other words, that the 

 recent species are more closely related to the original forms than 

 the forms of a half-way intermediate period — are we not justified 

 in assuming that there is here a reversion, or tendency towards a 

 reversion, to specific characters once lost ? Among the Foraminifera 

 we have several notable instances of apparent speciSc longevity, as, 

 for example, the recurrence in the modern seas of some of the cre- 

 taceous species of Globigerina, Cristellaria, and Glandaria ; and, not 

 unlikely, seeing how very slow must of necessity be the variation 

 in this class of animals, by reason of their ready adaptability to their 

 surroundings, the same forms will, on future investigation, be found 

 to date considerably further back in the geological scale. 



It must not be construed, from the preceding argument, that 

 we have attempted to prove the frequency of specific reappearances. 

 These, if they actually have existed, which is not unlikely, were, 

 doubtless, of exceptional occurrence, and in no way affect the prob- 

 lem of progressive development. It might be objected that, if the 

 views here set forth are correct in their application to species, they 

 must also apply in a corresponding degree to some of the higher 

 animal groups — ^genera, for example. This is certainly true, and, 

 indeed, it ipay well be asked, What insuperable obstacle is there, 

 that we know of, that should absolutely prevent the occasional re- 

 appearance of a lost genus ? Assuming, with many paleontologists, 

 that Goniatites is a genus evolved from Nautilus, either directly or 



