It is often said, but with some looseness of statement, that ancient quadrupeds are huger than their modern representatives. This statement is partly true in fact, but largely wrong in implication. For it suggests that--and the suggestion is often expressed in books that are not authoritative--huge animals {104} have left a dwarfish offspring; that there were giants of old, and that there is a puny race to-day. As a matter of fact, the study of the gradual evolution of the early Tertiary Mammalia into their descendants of later times shows very plainly the truth of this interesting generalisation: That the primitive types were all small creatures, and that in those instances where we can trace a pedigree, there was a gradual increase in size up to a point where greater increase led to extinction. We point out later on a number of facts illustrating this matter in detail. It has been ascertained, for instance, that the pedigree of the Horses, the Camels, the Rhinoceroses, and many other groups, commences with small forms and culminates in large ones. It may be urged that such animals as the Tapir are to-day smallish forms, and that related to them in the past were the gigantic Titanotheres; but in this and similar cases it will be found that the extinct giants were not in the direct line of pedigree, but represented side-branches which waxed huge on their own account and then disappeared. * * * * * {105} CHAPTER V THE EXISTING ORDERS OF MAMMALS PROTOTHERIA--MONOTREMATA Apart from those creatures whose fragmentary remains have been considered in the last chapter, and which belong to the earliest of mammaliferous strata, the remains of Mammalia are all referable to existing orders. In the pages which follow we shall therefore deal with the actual representatives of living families side by side with their extinct relatives. The existing orders of Mammalia, together with those of their fossil allies, can be plainly divided into two great subdivisions, or, as we shall term them, sub-classes; the Mammalia as a whole being termed a class of the Vertebrata comparable with the class Reptilia, etc. It has been usual, owing to the initiative of Professor Huxley, to divide the Mammalia into three divisions of primary importance. We shall adduce reasons later for not accepting this mode of division, but that which allows of only two primary divisions. These two divisions are (1) Prototheria and (2) Eutheria. Whether the Multituberculata, Trituberculata, and Triconodonta, considered in the last chapter, are really to be distributed among these two sub-classes is a matter upon which it is possible to form an opinion, but not to dogmatise. The Prototheria stand at the base of the mammalian series, and present many likenesses to the Sauropsida; the Eutheria are the animals which are most fully differentiated as mammals. We shall commence with SUB-CLASS I.--_PROTOTHERIA._