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♦ KNOWLEDGE • 



[Apbil 21, 1882. 



FOUiND LINKS. 



r.\ Im. Ankiikh Wii.suN, K.11.S.E., F.L.S. 

 I'AHT VI. 



THE iiiiadrupwls — or Muinnials, as thfy are technically 

 cjillwl- form an important group of animals, not 

 u.TiIy Ix-cjiust! in structure tlicy represent the perfection 

 i.f tin- uniiiial woriii, but because they stand at the liead of 

 the animal creation, apj)arently sejiaratt! and distinct from 

 all other and lower classes. The distinctive nature of the 

 (juadrupeds, in fact, has been tacitly acknowledged in 

 zoology in the Kyst«"ms of classification which themselves 

 uro mere expressions of thc> varied relationsliips of the 

 classified beings. For, whilst the fishes and frogs have 

 been united to form u province of Vertebrate animals, and 

 whilst reptiles and birds have also been arranged in one 

 chief group by rea.son of their affinities, the ijuadrupeds 

 h.ive been made to foi in a province by themselves. The 

 hairy nature of the body-covering, the nourishment of the 

 young by means of milk, the fact that the young are bom 

 alive, and many other characters well known in popular 

 zoology, attest the distinctive nature of the highest group 

 of animals. 



But whilst these statements cannot be questioned, it 

 must not be imagined that the quadrupeds are tliereby en- 

 tirely separated from all other animals. On the contrary, 

 they possess their own atlinitics with lower forms, such as 

 evolution pre-sujiposes, and such, indeed, as that theory of 

 nature demands. The lowest mammals, to begin with, are 

 by no means like the higlier quadrupeds ; and it is in the 

 lowest confines of the class, as we shall presently see, that 

 the approach to lower animals is made. The warmth of 

 blootl so characteristic of (juadrupeds has already made its 

 appearance in the birds, and although the e.\act origin of 

 the inaminals is yet a matter of doubt, it seems pretty 

 clear that the root-stock of the class to which man himself 

 belongs, may be sought for in some common territory 

 whence, from a half-bird type, the lowest quadrupeds 

 arose, or whence the mammals on the one side, and birds 

 and reptiles on the other, have independently arisen. Such 

 a conclusion seems to be that at present supported by facts 

 as they stand ; and although further research may modify 

 this view, there will still exist the demand for the links 

 that bind the quadrupeds to their lower Vertebrate 

 neighbours. 



There can exist, at least, no doubt of the remarkable 

 likeness which the lowest quadrupeds present to the bird 

 and reptile groups. To understand thorouglily the zoological 

 aspects of the matter, I may remind the reader that the 

 class of mammals is very sharply split into two main 

 dinsions. These, to avoid technicalities, we may term 

 Higher and Lower Mammals. The former group includes 

 forms ranging from man downwards through the apes, bats. 



Fig. 1.— Ornitliorhynchug, or " Duck-billod Watcr-Molo of 

 Aiutrmlin" (showing tlio "bill" and webbed feet). 



rodents, and hoofed fpiadrupeds, to the whales, sloths, 

 antcaters, and their kith and kin. These animals are dis- 

 tinguished )»y the higher brain-structure and by the general 

 possession of all the typical characters of quadrupeds. The 



Lower Mammals are the OrnithorhynchuB, or " Duck-Vnlled 

 Water Mob' " of Australia (Fig 1), and its neighl^ours the 

 Echidnas or " I'orcujjine Anteat<'rs " of Australia ; those 

 two genera forming the lowest order (Afcnwlroiinln) of all. 

 .\ little above them, but still sliut otf from the higher ranks 

 of the class, are the kangaroos, wombats, phalangers, ic. 

 — in a word, the whole iiative population of Australia 

 (along with the New World opossums), forming the order 

 of Marsiipiftliii, or that of the " pouched " quadrupe<ls. 

 In Fig. li is represented the pelvui or haunch-bones of a 

 kangaroo. At n, a, tlic Afaraupial botes, or those support- 

 ing the well-known pouch, are seen. These bones are only 

 found in the Marsupials and Monotremes, and whilst in 

 most of the foriiier they support a poucli, they are never 

 associated with that structure in the Monotremes. 



Now, it is in the Monotremes — represented liy the Orni- 

 thorht/ricJiun (Fig. 1) and the i'c/iidnas — that the characters 

 linking quadruped life to lower life are most typically seen. 

 It may be well to strengthen our position at the outset, by 

 reminding the reader that in the early life of all quad- 

 rupeds, without exception, there are to be perceived 

 evidences of their connection with lower forms of life. 

 Thus, every Vertebrate, at an early stage of its develop- 

 ment, exhibits certain clefts or openings in the sides of the 

 neck, knowni as branchial clefts, and which are bounded by 



Fig. 2. — Hauncli bones of Kangaroo : a, a, marsupial bones ; 

 d, socket for thigh-bone. 



folds called branchial arches. These, in fishes, come to bear 

 the gills, but in reptiles, birds, and quadrupeds they 

 simply disappear — useless rudiments of structures, once 

 necessary in the life of a(iuatic quadruped-ancestors, 

 and still retained in the developments of to-day by 

 the law of inheritance. Thus, in the development of a 

 raV)bit, the biologist sees three pairs of branchial arches 

 behind the mouth of the embryo, and four branchial clefts. 

 Three of the clefts disappear, and the fourth, by the modi- 

 fication to wliich development has been suVyected in the 

 evolution of the quadruped tribes, is converted into the 

 Eustachian tube and other structures belonging to the ear. 

 The presence of " branchial clefts " in the developing 

 mammal would alone suffice to show its evolution from 

 lower life. Denying that probability, which to the biolo- 

 gist is a fact, there is no explanation whatever of the cause 

 or exist<Mice of these vanishing structures in the history of 

 the quadruped race. 



Concentrating our attention on the " Monotremes " 

 themselves, however, we may speedily discover numerous 

 links which unite them with lower life, and specially with 

 the bird-typo. There, firstly, exists in these quadrupeds 

 what Iluxloy has called "a striking feature" of reptiles, 

 of birds, and of the frog-class as well, in the structure of 

 the shoulder. In the shoulder of an ordinary quadruped, 

 and of a kangaroo and its marsupial race as well, there are 



