No. 4. J DAWSON — POST -PLIOCENE. 407 



any of the 205 species of the above lists. This appears to me a 

 fact of extreme sio-nificance with reference to theories of the 

 modification of species in geoh>gical time. No geologist doubts 

 that the Post-pliocene was a period of considerable duration. 

 The great elevations and depressions of the land, the extensive 

 erosions, the wide and thick beds of sediment, all testify to the 

 lapse of time. The changes which occurred were fruitful in 

 modifications of depth and temperature. Deep waters were 

 shallowed, and the sea overflowed areas of land. The tempera- 

 ture of the waters changed greatly, so that the geographical dis- 

 tribution of marine animals was materially afiected. Yet all the 

 Post-pliocene species survive, and this without change. Even 

 variable forms like the species of Bitccinum and Astar-te show 

 the same range of variation in the Post-pliocene as in the mo- 

 dern, and though some varieties have changed their geographical 

 position, they have not changed their character. This result is 

 obviously independent of imperfection of the geological record, 

 because there is no reason to doubt that these species have con- 

 tinuously occupied the North x\tlantic area, and we have great 

 abundance of them for comparison both in the Post-pliocene 

 and the modern seas. It is also independent of any questions 

 as to the limits of species and varieties, inasmuch as it depends 

 on careful comparisons of the living and fossil specimens ; and 

 by whatever names we may call these, their similarity or dissimi- 

 larity remains unaffected. We have at present no means of 

 tracing this fauna as a whole farther back. Some of its mem- 

 bers we know existed in the Pliocene and Miocene without spe- 

 cific difference ; but some day the middle tertiaries of Greenland 

 may reveal to us the ancestors of these shells, if they lived so 

 far back, and may throw further light on their origin. In the 

 meantime we can affirm that the lapse of time since the Pliocene 

 has not sufficed even to produce neW races ; and the inevitable 

 conclusion is that any possible derivation of one species from 

 another is pushed back infinitely, that the origin of specific 

 types is quite distinct from varietal modification, and that the 

 latter attains to a maximum in a comparatively short time, 

 and then runs on unchanged, except in so far as geological 

 vicissitudes may change the localities of certain varieties. This 

 is precisely the same conclusion at which I have elsewhere 

 arrived from a similar comparison of the fossil floras of the De- 

 vonian and Carboniferous periods in America. 



