THE MAMMALIA. 



with respect to the surrounding medium, and one 

 that has undergone no other modifications than 

 those affecting the dimensions of the parts. 



In the other terminal members of mammals the 

 connecting skin extends no further than the roots 

 of the fingers or toes; but remains of this connect- 

 ing skin are still found in almost all mammals. 



Then a second fact is likewise conspicuous. 

 The first digit, the thumb or great toe, takes from 

 the very first a different direction from the other 

 digits. This divergence is maintained in most 

 mammals, and in addition to that the fact that this 

 digit has only two instead of three small bones or 

 phalanges contributes to give to it a peculiar 

 position. From a thumb having this difference in 

 direction as compared with the other digits to a 

 true thumb capable of being opposed to the other 

 digits, as in the human hand, there is no very great 

 interval, and we have moreover numerous examples 

 of intermediate forms. Besides, all limbs with 

 opposable thumbs or great toes have five digits, 

 and have accordingly preserved the original number. 



The Hand. — From these facts we infer that the 

 limbs with the first, digit standing apart from the 

 others represent the original form, and that those 

 with an opposable first digit, in one word, hands, 

 arc a very old type. We find, in fact, hands on 

 the hind-limbs in opossums and phalangers, lemurs 

 and monkeys; and in the two latter groups we 

 have in addition hands on the fore-limbs, which are 

 the only ones that persist in man. Although the 

 hand is indeed a wonderful instrument, it is never- 

 theless a very old pattern. How could the 

 marsupials just named, which are among the lowest 

 mammalian types, possess hands if these members 

 represented the last and highest stage of develop- 

 ment of the limbs.' 



If then a five-toed foot with the first toe standing 

 apart from the others is the primitive form, one 

 that we meet with in the oldest mammals known 

 to us, all the other forms of limb, often very much 

 modified, must necessarily have originated in this 

 fundamental type. 



This modification has been brought about mainly 

 by the degeneration of separate parts beginning 

 with the digits and proceeding upwards, but is also 

 due in some degree to the excessive development 

 of other parts. For details regarding this matter 

 we refer the reader to what is said with reference 

 to the separate orders. One will be able to judge 

 of the enormous influence which has been exercised 

 by the processes of reduction and fusion (the entire 



loss of certain bones, and the merging of two or 

 more into one) by comparing two extremes, for 

 example, the fore-foot of a porcupine ant-eater 

 (Echidna) and that of a horse. The former is five- 

 toed and has the full number of bones, namely 

 thirty-three; the latter is single-toed, has fused 

 and degraded bones, and the total number of the 

 bones is only seventeen, and two of these (two 

 bones belonging to the metacarpus) are functionless. 



The process of reduction can in this case be 

 traced both in the embryo and in ancestral forms, 

 which lay buried in past geological strata; and 

 from all these facts we may draw the general 

 conclusion, that all mammals with reduced or 

 degraded digits must be descended from ancestors 

 with complete five-toed feet, and that these modified 

 feet are the result of a longer or shorter series of 

 successive variations. 



The Position of the Limbs in Walking: 

 Digitigrada and Plantigrada — One last point 

 still remains to be explained. The limbs carry the 

 body, but they carry it in various ways. The 

 ruminants as well as the horses, the cat tribe as 

 well as dogs and their allies, place only the points 

 of their toes on the ground; they are Digitigrada. 

 The apes and monkeys, the Prosimii (lemurs, &c.), 

 bears, most of the marsupials and the monotremes 

 touch the ground with the entire sole, including 

 even the wrist and ankle; they are Plantigrada. 

 In certain orders we can follow the process of 

 raising the sole above the ground step by step, so 

 that the designations Semi-plantigrada and Semi- 

 digitigrada have been adopted for certain stages 

 in this process, although no great value can be 

 assigned to these sub-divisions. Yet the fact itself 

 does not thereby lose its importance, for all the 

 old Eocene mammals are plantigrade, whether 

 Carnivora or Ungulata, Perissodactyla or Artio- 

 dactyla, Insectivora or Rodentia. From this we 

 conclude that all the Digitigrada have had planti- 

 grade ancestors, and that the plantigrade mode of 

 progression, such as is found in man himself, must 

 be regarded as a primitive character which has 

 been retained. 



The Teeth. — One of the most complicated 

 chapters in the natural history of the Mammalia is 

 that which relates to the teeth, and we readily con- 

 cede that it pre-sents difficulties that might deter 

 more than one reader who did not wish to make 

 any special study of the subject. Although we are 

 far from wishing to disguise these difficulties, yet 

 we must take the subject resolutely in hand, for 



