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♦ KNOWLEDGE ♦ 



[June 1, 1887. 



birds and reptiles; the heart is at first a simple pulsating 

 chamber like that of worms ; the back-bone is prolonged 

 into a movable tail ; the great toe is extended, or opposable, 

 like our thumbs and like the toes of apes ; the body three 

 months before birth is covered all over with hair, except on 

 the palms and soles, like the soles of four-footed animals. 

 At birth the head is relatively larger and the arms rela- 

 tively longer than in the adult ; the nose is bridgeless ; both 

 features, with others which need not be detailed, being dis- 

 tinctly ape-like. Thus does the egg from which man springs, 

 a structui-e only T^i-th of an inch in size, set before us 

 the history of bis development from fish-Uke and reptilian 

 forms, compressing into a few weeks the results of millions 

 of years. That which is individual or peculiar to his race, 

 the physical and mental character inherited, is left to the 

 slower development which follows birth. 



The gills in his embryo, as also in that of other mammals, 

 are one among many structures, more or less rudimentai-y, 

 which witness to the unity of origin of every living thing. 

 Certain organs appear in the fcetus which are useless to the 

 adult, as teeth in whales, remnants of hind limbs in certain 

 snakes, wings under the wing cases of insects that do not 

 fly, rudiments of pointed ears in man, abortive stamens in 

 plants, as the snapdragon, and so forth. Except as evidence 

 of the modification of life-forms in which they occur from 

 other life-forms, and of persistency of type, they are mean- 

 ingless ; the functions they once discharged have long ceased, 

 being exercised only in other and allied living things where 

 they are found fully developed. 



II. Morpholoijy. — Large gi-oups of species, whose habits 

 are widely different, present certain fundamental likenesses 

 of structure. The arms of men and apes, the fore-legs of 

 quadrupeds, the paddles of whales, the wings of birds, the 

 breast-fins of fishes, are constructed on the same pattern, 

 but altered to their several functions. Xearly all mammals, 

 from the long-necked giraffe to the short-necked whale, 

 have seven neck-bones ; all insects and Crustacea — moth and 

 lobster, beetle and cray-fish — are alike composed of twenty 

 segments ; the sepals, petals, stamens, and pistils of a flower 

 are all modified leaves arranged in a spire. Such facts need 

 no comment. 



III. Classification. — It has been already shown that all 

 animals may be reduced to three types : the first, or lowest 

 and simplest, without body cavity ; the second, or inter- 

 mediate, with bod}- cavity : the third, ranging from the 

 earth-worm to man, with digestive cavity separate from 

 body cavity. The general likenesses of structure upon 

 which a classification of living things is based have been 

 detailed in the chapters on existing life forms,* and here 

 the reminder sufiices that the old attempts at a linear 

 arrangement have failed, and that the only true mode of 

 presentment, both of the life that is and that was, is that 

 of a tree with short trunk, indicating common origin of the 

 living from the non-living, and divided into two large 

 trunks representing plants and animals respectively. From 

 each of these start large branches representing classes, the 

 larger branches giving off smaller branches representing 

 families, and so on with smaller and smaller branches 

 representing orders and genera, until we come to leaves as 

 representing species, the height of the branch from which 

 they are hanging indicating their place in the growth of the 

 great life-tree. 



IV. Succission. — All extinct forms have their time-range, 

 all living forms have their space-range, and, if the theory 

 of descent has any truth in it, their ancestors must be sought 

 in the past. Broken as is the record, the great mass of it 

 beyond reach, and the little that is witliin reach only 



* \1de Knowledge, June to October, 1886. 



thoroughly examined here and there, the accord of the facts 

 of geological distribution with the facts summarised above 

 is nevertheless complete. Each formation has its peculiar 

 groups of fossil remains representing the life-forms of the 

 period ; the older the rock, the simpler are its organic 

 remains, and, what is of no mean importance, although 

 transitional forms are from their nature fewer and less per- 

 manent than forms which have arrived at balance with their 

 surroundings, the fossil-yielding rocks have disclosed the 

 existence of several hitherto missing links between species. 

 Reference to some of these has been made in the summary 

 of past life-forms, e.g., to the proofs of descent of the one- 

 toed horse of to-day, with his knee corresponding to our 

 wrist or ankle, from the five-toed primitive horse found in 

 the Eocene beds of North America, and to the connecting 

 link between birds and reptiles supplied by the Arch,-?opteryx. 

 To this may be added the t'ompsognathus, with its swan-like 

 neck, its toothed jaws, and hind limbs, on which it walked. 

 Then there are the links between pigs and ruminants in 

 the Anoplotherium ; between tapirs, horses, and rhinoceroses 

 in the Pala>otherium ; and in the Devonian strata forms 

 occur which are considered intermediate between ganoids and 

 mud fishes. Thus one by one the blanks are being filled 

 up ; the faith of the biologist is justified by his works. 



V. Distribution. — Every living thing has its definite 

 area of range : the sloth is peculiar to America, the 

 hippopotamus to Africa, the chamois to the Alps ; Arctic 

 plants wither under the equator ; those of the tropics 

 perish in cold or even temperate zones, while a vast number 

 flourish only in the original birthplace of all life — water. 

 Among animals a few, notably man and the cat genus, have 

 spread themselves well nigh everywhere ; but, as a rule, cer- 

 tain species are restricted to certain regions, and hence the 

 biological division of the land into regions corresponding to 

 that distribution, and of the water into life regions measured 

 by the limits of depth at which marine forms are found. 

 Speaking broadly, the plants and animals of countries in 

 unbroken connection resemble one another, while those of 

 countries remote or cut off are unlike. But these general 

 principles bristle with exceptions. In countries where the 

 climate and general conditions correspond, as the equatorial 

 regions of both the Old and New World, as ahso on 

 the same continents, there are marked differences in the 

 life-forms, probably owing to impassable barriers of mountain 

 ranges, deserts, or oceans, while in countries with different 

 climates, as in tropical Florida and frozen Canada, allied 

 types are sometimes found. Puzzling and capricious as the 

 distribution of life may seem — e.g., tapii'S are found only 

 in South America and Mrdacca, with its neighbouring 

 islands, being thus separated by nearly half the globe's 

 circumference — in this, as in aught else in nature, nothing 

 is accidental. Distribution is due to the migration and 

 transport of living things, which, under the agency of 

 natm-al selection, become more or less adapted to new con- 

 ditions, and much light comes therefrom, not only on the 

 theory of the origin of species, but also upon p:ist changes 

 in the relations of land and water. Where allied forms 

 which are unable to cross the seas are found in lands now 

 separated, as in Britain and Japan, in Southern Europe and 

 North Africa, we have evidence of former union ; the degree 

 in which species have been modified giving some key to the 

 remoteness of that union. 



In the study of the very complex problem of distribution, 

 isl.ands afford important aid. They are of two kinds, con- 

 tinental and oceanic. The continental have been broken 

 off from the mainland, as the British Isles, Japan, the far 

 more ancient Madagascar with its lemxirs, and New Zealand 

 with its wingless birds and remarkable one-eyed lizard. 

 The oceanic, as the Azores and Sandwich Islands, are of 



