Ml LECTURES ON* EVOLUTION m 



present time. That is the case with one of 

 th<* cretaceous lamp-shells (Terebratula), which 

 has continued to exist unchanged, or with insigni- 

 fir.mt variations, down to the present day. Such 

 is the case with the Globigerince, the skeletons of 

 which, aggregated together, form a large propor- 

 tion of our English chalk. Those Gloligerince can 

 be traced down to the Globigerince which live at 

 the surface of the present great oceans, and the 

 remains of which, falling to the bottom of the sea, 

 give rise to a chalky mud. Hence it must be 

 admitted that certain existing species of animals 

 show no distinct sign of modification, or trans- 

 formation, in the course of a lapse of time as 

 great as that which carries us back to the Creta- 

 ceous period ; and which, whatever its absolute 

 measure, is certainly vastly greater than thirty 

 thousand years. 



There are groups of species so closely allied 

 together, that it needs the eye of a naturalist to 

 distinguish them one from another. If we dis- 



ird the small differences which separate these 

 forms, and consider all the species of such groups 

 as modifications of one type, we shall find that, 

 even among the higher animals, some types have 

 had a marvellous duration. In the chalk, for 

 iple, there is found a fish belonging to the 

 highest and the most differentiated group of 



us fishes, \\liieh goes by the name of Bcryx. 

 Tin- remains of that fish are among the most 



