Page - 377 - in The Origin of Species

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-