INTRODUCTION. 



15 



Can we fill up the gap b}- bupposing that there 

 arose at some period within it a single placental 

 t)'pe from which all the present forms are descended? 

 Or must we assume more than one primitive 

 placental stock to account for the present diverg- 

 ences among the members of the class? The 

 answers to these questions must necessarih' be 

 based largely on speculation, and though the facts 

 that have been ascertained may justify pretty 

 confident conclusions ■with regard to some of them, 

 we are ine\itably restricted to hypothesis A\-hen we 

 attempt to trace the history or evolution of an}- 

 particular group. 



So much, ho'wex'er, is certain. After the long 

 gap bet^\•een Jurassic and Tertiary times there 

 appear \\'ith the Eocene both in the Old World 

 and the New the following still surviving orders: 

 Prosimii, Insectivora, Carnu'ora, Perissodact}'la, 

 Artiodact)'la, Rodentia, and in the Old World alone, 

 the Chiroptera. All these orders are represented 

 either by families -which are now quite extinct, or 

 by equivocal tj^pes which cannot be referred to an}- 

 famil}', or, lastly, by forms still existing. Thus the 

 Prosimii in France have furnished a genus allied to 

 the Potto, while the other families are extinct. 

 Further, leaf-nosed and smooth-nosed forms are 

 found among the bats; tanrecs, moles, and shrews 

 among the Insecti\ora ; Canida and Viverrida 

 among the Carnivora; tapirs and horses among 

 the Perissodact)-la; pigs among the Artiodact)-la; 

 squirrels, dormice, mice, degus (genus Octodon), 

 and spiny rats (Echimyida) among the rodents, 

 together with extinct families and indeterminate 

 types, from which families better characterized can 

 be deri\-ed. 



This accordingly is the oldest nobility among 

 the placental mammals, whose family trees can be 

 traced back to the Eocene. The roots manifestl)- 

 reach much further back, but we have no certain 

 knowledge of them. 



Let us now consider this fact in the light of the 

 present distribution of mammalian forms. Seeing 

 that all these orders in families alread)' existed in 

 Eocene times, if there is an}- region of the globe 

 from which the_\- are altogether absent, or in which 

 they are poorl\- represented, that deficienc}- must 

 be accounted for. It is universalh' found that when 

 a higher t}-pe appears it tends to displace the lower. 

 The latter becomes extinct, or gets reduced to a 

 few representatives enabled b}- special circumstances 

 or habits to survive. The}- ma}- escape even amidst 

 the competition of higher forms through the fact of 



their li\-ing on trees, and thus being out of the 

 reach of man}- enemies, or through being able to 

 hide in burrows, or being nocturnal in their mode 

 of life. But, apart from such special favouring 

 conditions, the doom of the lower t\-pe is to dis- 

 appear before the higher, and since the higher 

 placental forms were alread}- largcl}- represented 

 in Eocene times, \\-e must ask how it is that an}- 

 region of the earth has escaped being overrun b}- 

 them. 



Such a region does exist in Australia and some 

 of the neighbouring islands. The fauna of that 

 region comprises but few placental forms among 

 its mammals, but a great number and variet}' of 

 such as have no placenta. There is onh- one wa}- 

 of accounting lor such a fact as this. Before the 

 appearance of the chief representatives of the 

 placental t}-pe, that is to sa}-, before the Eocene 

 epoch, the Australian region must have been cut 

 oft' from the other lands of tlie globe; it must thus 

 have been saved from that struggle for existence 

 in which the higher forms of the Eocene epoch 

 took part. 



Passing on now to the Miocene we find that in 

 that epoch the number of the orders is completed. 

 None of the orders of the present da}- is wanting 

 in Miocene formations. ]\Ionke}-s, seals, whales, 

 sea-cows, elephants, and edentates all appear. The 

 families become specialized, the monke}-s of the 

 Old and New \A'orld, hedgehogs, felines, h}-a:nas, 

 martens and bears, rhinoceroses and hippopotamuses, 

 as well as ruminants, can be more and more clearly 

 discriminated. In short we see how in the strata of 

 the Miocene the t}-pes follow one another in higher 

 and higher degrees of specialization. 



It is clear, moreover, that those regions which 

 present to our view onl\- the orders appearing 

 first in the Eocene, and -\\-hich are -\\-ithout the 

 orders originating in the [Miocene, -were isolated 

 before the latter epoch, though not till after the 

 Eocene. 



In this position are [Madagascar and the Antilles. 

 The great African island is mainh- inhabited b}- 

 prosimians; but there are found in addition bats, 

 insecti\-ores, rodents, a species of pig, Aix'errines, 

 and the old t}-pe Cr}-ptoprocta. No \-estiges, how- 

 ever, have }-et been found there of an}- order or 

 family belonging to the iMiocene — neither monke}-s, 

 nor ruminants, nor edentates, ^\■hich are all so 

 abundant, nevertheless, both on the mainland of 

 Africa and in India. The Antilles present an 

 analogous case, diftering onl\- in the details. The 



