106 AFFINITIES OF EXTINCT SPECIES. [Chap. XL 



general parallelism 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 them- 

 selves 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 tertiary formations. Barrande, also, shows that 

 there is a striking general parallelism in the successive 

 Silurian deposits of Bohemia and Scandinavia ; never- 

 theless he finds a surprising amount of difference in 

 the species. If the several formations in these regions 

 have not been deposited during the same exact periods, 

 — a formation in one region often corresponding with a 

 blank interval in the other, — and if in both regions the 

 species have gone on slowly changing during the accu- 

 mulation of the several formations and during the lon^ 

 intervals of time between them ; in this case the several 

 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 



