508 CHORDATA. 
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 Zutheria 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 
Lutheria, 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 Sirenta and Cefacea are entirely aquatic or nata- 
torial, the Chiroptera are entirely erial, 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 affect the primitive terrestrial, arboreal, or 
fossorial habitats usually with nocturnal proclivities. In- 
stances of this may be seen in the Zdentata, [nsectivora, 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 :— 
T. Rabbit oe ccvescica various Primitive terrestrial. 
2-3. Horse and Ox,............ Cursorial. 
4-5. Dog and Cat, ............. Transition cursorial. 
6: ‘Sloth: owsses- essere ian ceseee Arboreal. 
y's. MOLGS way oseroneonees nes ean te ane Fossorial. 
8. Porpoise, ....Natatorial. 
Gi: TBAty cin estaeeehaenacs ant es ferial. 
1. The Rabbit has already been described. 
1. 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 /isectzvora of the present 
day probably give us an approximate semblance of them. They were 
