AFFINITIES OF EXTINCT SPECIES 377 



the forms of life; but the species would not exactly corre- 

 spond ; for there will have been a little more time in the one 

 region than in the other for modification, extinction, and 

 immigration. 



I suspect that cases of this nature occur in Europe. Mr. 

 Prestwich, in his admirable Memoirs on the eocene deposits 

 of England and France, is able to draw a close general par- 

 allelism between the successive stages in the two countries; 

 but when he compares certain stages in England with those 

 in France, although he finds in both a curious accordance in 

 the numbers of the species belonging to the same genera, yet 

 the species themselves differ in a manner very difficult to 

 account for considering the proximity of the two areas, — ■ 

 unless, indeed, it be assumed that an isthmus separated two 

 seas inhabited by distinct, but contemporaneous, faunas. 

 Lyell has made similar observations on some of the later ter- 

 tiary formations. Barrande, also, shows that there is a strik- 

 ing general parallelism in the successive Silurian deposits of 

 Bohemia and Scandinavia; nevertheless he finds a surprising 

 amount of difference in the species. If the several forma- 

 tions in these regions have not been deposited during the 

 same exact periods, — a formation in one region often cor- 

 responding with a blank interval in the other, — and if in 

 both regions the species have gone on slowly changing dur- 

 ing the accumulation of the several formations and during 

 the long intervals of time between them; in this case the sev- 

 eral formations in the two regions could be arranged in the 

 same order, in accordance with the general succession of the 

 forms of life, and the order would falsely appear to be 

 strictly parallel ; nevertheless the species would not be all 

 the same in the apparently corresponding stages in the two 

 regions. 



ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER 

 AND TO LIVING FORMS 



Let us now look to the mutual affinities of extinct and 

 living species. All fall into a few grand classes; and this 

 fact is at once explained on the principle of descent. The 

 more ancient any form is, the more, as a general rule, it dif- 



