ioo THE LESSON OF EVOLUTION 



rise to new groups ; and these unspecialised forms are 

 usually small. The specialised forms, which answer 

 to the leaves in the analogy, die out, because, when a 

 change in their habits becomes a necessity, they cannot 

 change their bodily shape. This is not a proof of 

 degeneracy, or of decline in vigour. When man takes 

 these highly specialised plants or animals in hand, 

 and domesticates them, they shew no sign of loss of 

 vigour ; and in time they regain the power of variation. 



It is generally acknowledged that the sudden 

 appearance of a new group, in large numbers and in 

 considerable variety, is due to migration ; and this 

 implies that the group was developed in some district 

 which is now either inaccessible or unexplored. This 

 can be the only explanation of the sudden appearance 

 of eutherian mammals in the lower Eocene rocks of 

 North America and Europe ; and of the graptolites in 

 the lower Ordovician of Europe. The appearance of 

 dicotyledonous plants in the lower Cretaceous of North 

 America and Europe is more difficult to explain. This 

 flora is thought by some to have originated in the arctic 

 regions, and to have spread southwards ; but, how- 

 ever this may be with the deciduous trees, a varied ever- 

 green dicotyledonous flora existed in New Zealand with 

 Belemnites and marine Saurians (Cimoliosaurus, Liodon, 

 &c.), which can hardly be younger, and may perhaps 

 be older, than the cretaceous flora of Greenland. 



From what I have said above in the Preface, my 

 readers will understand that this second Essay is to be 

 taken as a kind of appendix to the first, supporting 

 and illustrating the theory therein contained, by a 

 statement in outline of the facts of biological evolution 

 in so far as they have hitherto been ascertained. 



