348 



♦ KNO^A^LEDGE ♦ 



[October 1, 1886. 



perch of India, can also leave the water, but their breathing 

 is effected by special organs connected with the gills. 



Here, then, we find the intermediate step between land- 

 dwellers and water-dwellers, the most perfect and fomiliar 

 example of which is supplied by the common frog's life- 

 history. The gill-breathing, limbless, tailed, plant-eating, 

 ai{uatic tadpole develops into the lung-breathing, four- 

 legged, web-footed, tailless, animal-eating, amphibian frog, 

 unable, save when torpid, to live in water without coming 

 to the surface for aii-. Some amphibians possess both lungs 

 and gills throughout life, but all the higher vertebrates, 

 whether they live in water or not, breathe through lungs, 

 which arise, like the air-bladder of fishes, as sac-like out- 

 growths of the primitive gullet. 



Heptiles, which include forms as diverse as the nimble 

 lizard, the sluggish crocodile, and the limbless snakes, are 

 for the most jiart the relatively insignificant descendants of 

 the monsters of the land, air, and water, that flourished in 

 the age of reptiles amidst the dense vegetation of a swampy 

 world until conditions ftital to them, and favourable to the 

 development of higher life-forms, supervened. 



Although the exact sequence of birds in the appearance of 

 vertebrates is unknown, their descent from reptiles is certain. 

 Not, as might be thought, from the flying species, for these 

 were featherless and more like bats than birds in the mem- 

 braneous wings which stretched from limb to body, but 

 from species that walked upon the land. But, although 

 structural likenesses between birds and reptiles survive to 

 attest this former close relation, as e.g. the union of the 

 skull to the spine by a single joint, instead of by two joints 

 as in most amphibians and in all mammals, and the union of 

 the skull to the jaw by the quadrate bone, which enables 

 the jaws to be opened very widely, manifold causes, work- 

 ing through long jjeriods, have brought about marked dif- 

 ferences of external and internal structure. Notable among 

 these is the modification of the three- chambered heart of 

 nearly all reptiles into the four-cbambered heart of birds 

 and mammals, by which the fresh and used-up blood are 

 kept separate, and the higher temperature of the body 

 maintained. The scales of the one, and the feathers or 

 furry covering of the other, are alike modifications of the 

 outer skin ; for although the intermediate stages between 

 the plumage of bu-ds and the horny plates of reptiles are 

 missing in fossil remains, it is certain that these and kindred 

 structures, as well as claws, nails, and hoofs, aie all out- 

 growths of the skin ; even the teeth, the variety of form 

 and arrangement of which renders them of gi'eat value in 

 determining the habits and general structure of the animal 

 to which they belonged, being secreted from the skin. 

 It has been shown already that the nerves are also formed 

 from the skin, nor should the variety of function which it 

 discharges, and therefore the variety of structure into which 

 it is modified, surpri.se us when we reflect how continuous 

 has been the action of the medium with which it is in 

 immediate contact uijon the external surface of all organisms, 

 so that the slight film or integument of the lowest has 

 developed into the complex layers which enclose the highest. 



Space prevents enlarged reference to the significant 

 proofs of the development of birds as compared with their 

 reptilian ancestors in larger proportion of brain to body, 

 with the intelligence which this connotes, a remark- 

 able proof of which is in the wide range and method of 

 their migrations, involving powers of vision and memory 

 exceeding that possessed by man. On this matter 

 Darwin remarks : " How a small and tender bird coming 

 from Africa or Spain, after traversing the sea, finds the 

 very same hedgerow in the middle of England, where it 

 made its nest last season, is truly marvellous." 



The lowest members of the diversified group of Mammals 



or milk-rjivers resemble birds in being toothless, and in 

 having a common sac into which the intestines and other 

 oi'gans open, for which reason they are called Moiiotremes or 

 one-vented. These animals are represented by the Ornitho- 

 rhynchus,* or DucJc-biU, a beaver-like creature with a horny 

 bill, and feet furnished with both webs and claws, and by 

 the Echidna, or Spin// AtU-eatei-, which resembles a large 

 hedgehog, being snouted and covered with jn-ickles. Each 

 is found in Australia, that land of primitive forms, and 

 recent discoveries invest them with the greatest importance 

 as links in the chain of mammalian descent. P'or they both 

 lay eggs like i-eptiles and birds, the duck-bill laying two at 

 a time, which she deposits in her underground nest, and the 

 echidna laying one, which is probably hatched in her pouch. 

 And the eggs further correspond with those of birds and 

 reptiles in having not only the yolk from which the embryo 

 is formed, but the yolk-sac containing the food on which it 

 is nourished until hatched, when it lives on the milk which 

 the mother pumps from her teatless glands until it can 

 shift for itself. Now an animal that unites in itself these 

 reptilian and mammalian features is to be classed among 

 the interesting anomalous and intermediate forms which 

 Darwin has happily termed " living fossils." Whether 

 monotremes are descended in direct line from reptiles, with 

 the internal structure of which they have much in common, 

 and mammals from monotremes ; or whether there was an 

 ancestral form or root-stock from which both reptile and 

 mammal branched off, so that mammals are as old as rep- 

 tiles, and older than birds, is not clear, although the rocks 

 may one day reveal it. But the interrelation of reptiles and 

 mammals is proven beyond question. 



The next stage in mammalian development is marked by 

 the 3Iarsupials or pouched milk-givers, as kangaroos and 

 opossums, the young of which are born in an imperfect 

 condition, and nourished and kept in the mother's pouch 

 till they can run alone. Theii- fossil remains evidence to a 

 wide range in Triassic times, and the Post-Pliocene beds of 

 Australia yield bones of marsupials as large as elephants ; 

 but then- habitats are now limited to that island continent 

 and to lands similarly long isolated. 



In all other mammals the yoirng are born fully formed, 

 being attached during the time of their development within 

 the mother to a structure called the pUtcenta, thr'ough 

 which they are nourished by her, whence the general 

 term placental manimals. Starting thus more or less fully 

 equipped in the struggle for life, the chances in their 

 favour were incomparably greater than those of animals 

 which are precariously hatched or born in an imperfect 

 state ; hence, among other causes, the dominance of the 

 placentals and their development into the highest organisms 

 yet reached. 



They are usually divided into the following classes, which 

 indicate structural characters common to the animals in- 

 cluded under each, and not the relative place of any class 

 in the sub-kingdom. No linear arrangement of classes, nor, 

 sometimes, even of species, is possible, for the succession of 

 forms is not as that of steps of a ladder, but as of a many- 

 branched tree. 



1. Toothless . Sloths ; ant-eaters ; armadillos. These show 

 {Edentata) afEnities linking them nearer to mono- 

 tremes than to marsupials. 



2. SrRENS . . So-called from their fancied resemblance 

 {Sin-iiiii) to mermaids or sirens. Dugongs and 



manatees, or sea-cows ; fish-like in 

 form, the fore-iimbs modified into 

 paddles, the hind limbs absent ; both 

 of these are flesh-feeders. 



* OrnitkorhT/nchus paradoxus, the older naturalists named it, for 

 when they were assm-ed that the creature was not a fraud of the 

 stuffer, they thought it must be a freak of nature. 



