32 THE MAMMALIA [SECT. A 



molars have pointed cusps : incisors are never fewer than two in 

 the lower jaw on each side. The brain is very simple in con- 

 formation. Insectivora. 



Order 8. Eutheria in which the pectoral limbs are modified 

 to form wings, the bones being greatly elongated so as to support 

 a broad web of skin extending to the hind limbs posteriorly. 

 The ulna is vestigial, the clavicles well developed. Cheiroptera. 



Order 9. Eutheria with prehensile limbs (adapted to arboreal 

 life), the thumb and great toe being more or less completely 

 opposable to the other digits. The digits are nearly always five 

 in number, and are provided usually with flat nails : the clavicles 

 are well developed. The brain is variable, but may present high 

 conditions of development of the cerebral hemispheres. Primates. 



It is to be remarked that although numbered from 1 to 9, it 

 does not follow that this arrangement places the various Eutherian 

 Orders in sequence according to the morphological status, a subject 

 to be discussed in the next chapter. Here it is to be said that 

 the Edentata, Ungulata, and Insectivora as defined above are the 

 Orders most distinctly open to criticism. In other words, each of 

 these three Orders includes within its own limits animals which 

 can be admitted only on the plea that simplification is a prime 

 necessity for the purposes of study. We have just seen (p. 26) 

 that the most recent classification provides fourteen Orders instead 

 of the nine given above. And there is much evidence to support 

 such an increase. Yet the older system is found to be generally 

 useful, and it is applicable to a large number of mammals; its 

 retention is justified on these grounds as well as on account of its 

 simplicity. That it is after all largely conventional, although 

 based on the observed facts of animal structure, cannot be too 

 strongly urged ; but the advantage conferred by the mere fact that 

 we can classify mammals is great, for it enables us to compare their 

 forms and characters much more easily than if they were not sus- 

 ceptible to reduction to such order. But like conventional systems 

 of other kinds, it has its limits ; in other words, it does not apply 

 to every animal. For there are animals which, when examined in 

 the light of morphology, prove hard to fit into this classification. 

 When we meet with such an instance, we should therefore remember 



