5o8 CHORD AT A. 



this fact is not quite clear, but as the brain is the special 

 centre regulating interaction between the organism and its 

 environment, it is probably the structural expression of the 

 increasing '* complexity " of life now followed by higher 

 organisms. (See page 462). 



At the present day the Eutheria are tolerably sharply 

 differentiated into orders, but the energy of palaeontologists 

 has in recent times unearthed a number of transition forms 

 which, whilst adding enormously to the difficulties in the 

 way of a "natural" classification, enable us to trace the 

 descent of the greater number of our modern types. 



Adaptive modification is very conspicuous in the 

 Eutheria^ and, as elsewhere, it has taken place to a large 

 extent independently of genetic connection. At the same 

 time we find in several cases that the two are parallel. Thus 

 the orders Sirenia and Cetacea are entirely aquatic or nata- 

 torial, the Chiroptera are entirely aerial, and as a rule the 

 Primates are arboreal, though only of the " transition '' 

 group, whilst the true cursorial are mainly in the Ungulata. 



Again, we find as a general rule that the lower or more 

 primitive types aff'ect the primitive terrestrial, arboreal, or 

 fossorial habitats usually with nocturnal proclivities. In- 

 stances of this may be seen in the Edentata^ Insectivora^ a 

 number of Rodentia and the most generalised of the 

 Carnivora. 



We may illustrate the structure of the Eutheria by a 

 short study of the following types : — . 



I . Rabbit, Primitive terrestrial. 



2-3. Horse and Ox, Cursorial. 



4-5 . Dog and Cat, Transition cursorial. 



6. Sloth, Arboreal. 



7. Mole, Fossorial. 



8. Porpoise, Natatorial. 



9. Bat, y&ial. 



I. The Rabbit has already been described. 



I. The Primitive Terrestrial Types. — Hedgehog, Shrew and 

 Bear. — We must suppose that the first mammals were small generalised 

 terrestrial mammals, with tubercular teeth and omnivorous diet, inclining 

 to insects and/' small-flesh." They merge into the incidental arboreal, 

 fossorial and cursorial forms : some of the hisectivora of the present 

 day probably give us an approximate semblance of them. They were 



