284 GENERALIZATIONS. 



characteristics which are precisely the same*. If we examine 

 the formation immediately following any early period which 

 belongs to a new epoch, that formation may contain some spe- 

 cies inherited from the preceding period, but the greater part 

 of the species show us a new type, and present distinct charac- 

 teristics. 



Some common species may be found in the beds which sepa- 

 rate two geological periods ; but Prof. Heer has not noticed any 

 form which would indicate a fusion of the species. The new 

 forms contrast with the old ones as new money looks different 

 from worn coins. Under the influence of altered climate and a 

 change of locality, the new species may present numerous modi- 

 fications, called "varieties/'' or they may have more decided 

 variations, and give rise to " races \' 3 but in their intermixture 

 the species always produce fertile individuals, while the true 

 hybrids are generally barren. Although a species may deviate 

 into various forms, it nevertheless moves within a definitely ap- 

 pointed circle, and preserves its character with wonderful tena- 

 city during thousands of years and innumerable generations, and 

 under the most varied external conditions. 



Prof. Heer maintains that in nature there is exhibited much 

 less of a tendency towards the fusion of species than of a force 

 manifested to preserve specific characteristics. Hence cultivated 

 plants and domesticated animals show an inclination to return 

 to their original wild forms, and between species there is usually 



* The ' Bear-Island Flora ' (French edition, p. 20) is instructive on this 

 point. Almost all its species are identical with those of the lower formations 

 of the Carboniferous period in Europe. The most important species occur 

 also in the Greywackes of the Hartz and of Silesia, although these Gray- 

 wackes "belong to the upper stage of the Lower Carboniferous, whilst the 

 strata of Bear Island form part of the lowest stage, and the whole formation 

 of the Carboniferous limestone lies between them. Here, therefore, we have 

 species which have remained the same during an immense lapse of time, and 

 under very different external conditions. The Miocene flora of Spitzbergen 

 presents analogous facts. Its bald cypress is identical with the species 

 which now inhabits the Southern United States (Taxodium distichuni), al- 

 though it must have lived at Spitzbergen among quite different associates, 

 and under different external conditions. Professor Heer has made the same 

 observations with regard to the mountain-pine and the common fir (Pinus 

 Abies, Linn.). 



