74 THE LESSON OF EVOLUTION 



descended from common progenitors ; a proposition* 

 which only a few naturalists have doubted. If this 

 be assumed , it follows that life must have existed on 

 the earth during the whole of the specific life of any 

 particular .species ; and we can find a series of species 

 whose specific lives overlap one another, and so carry 

 on the inference from the Cambrian period up to the 

 present day. 



4. The more generalised types appeared before 

 those more specialised. A generalised type is one 

 that is related to several different groups, and is 

 thus distinguished from a specialised type, which 

 shews some marked peculiarity. Thus the early 

 reptiles are related to several of the different families- 

 which follow them. The Trilobites connect the 

 Crustacea with the Arachnida. The Eocene mam- 

 mals were not separated distinctly into the- 

 specialised groups that we find in the Oligocene and 

 Miocene ; and this can be carried into considerable 

 detail. 



5. The succession of forms of life is the same in 

 different parts of the world. In all regions Trilo- 

 bites, Graptolites and Spirifers have been succeeded 

 by Ammonites, Belemnites and Reptiles, and these, 

 again, by Birds and Mammals. So that there has 

 been a general and uniform advance in life on the 

 earth. 



Proof of evolution. Now all these generalisations 

 are explained by the theory of descent with modifi- 

 cation. That extinct animals and plants were con- 

 structed on the same plan as the living ones is be- 

 cause the latter are the descendants of the former. 



