October 1, 1886.] 



KNOW^LKDGE ♦ 



34^ 



almost bodily from the record of Crisbna), the Protevan- 

 geliou, or First Gospel, attributed to Jamas, the brother of 

 Jesus (before the doctrine of Mary's perpetual vuginity had 

 arisen ), gives a very full account of the birth of the child 

 in a cave, to which Joseph had been obliged to take JIary, 

 who had been seized with labour pains while on a journey, 

 and to his account the Catholic Church seems still to incline, 

 as any one may learn who will be at tlie pains to visit a 

 Catholic cathedi'al at Christmas time. 



THE STORY OF CREATION. 



a plain account of evolution. 

 By Edwakd Clodd. 



XI.— PRESENT LIFE-FORMS. 

 H Animals (concluded).— VI. Vertebrata. 



E now reach the last and highest of the 

 divisions of animal life — the sub-kingdom 

 of the back-boned, which includes Man" — 

 and when we note the differences between 

 its representative types and those of the 

 divisions below it, all of which include 

 bonekss animals, we feel that, if there be 

 any break in the continuity of life, it should 

 be found between invertebrates and vertebrates. 



Speaking broadly, since the lowest living things have no 

 unlikeness in parts, invertebrates juay be said to consist of a 

 single tube or cavity containing the nervous and vascular 

 systems in common, and to have an outside skeleton, which 

 is simply a hardening of the skin. In contrast to this, 

 vertebrates consist of two tubes or cavities, the smaller of 

 which encloses the central parts of the nervous system {i.c., 

 the brain and spinal cord), and the other the vascular system 

 {i.e., the organs of digestion and cu'culation), and they have 

 an inside skeleton, the most importmt part of which is the 

 spine or backbone, which separates the two tubes, and is 

 made up of a number of jointed bones or vertebrEe united by 

 remains of the cartilaginous notochord, thus giving iiexible 

 action to the whole column. The advantage of this com- 

 bined strength and ease of movement is seen in fishes as 

 they dai-t through the water, in the gliding of the snake, the 

 leap of the antelope, and the spring of the lion ; while, as 

 compared with animals which are either naked or covered 

 by a rigid homy skeleton, cumbersome as the armour of our 

 ancestors, vertebrates have an enormous superiority in their 

 internal framework of living bone, which adapts itself to, as 

 well a-s nourishes and protects, the softer parts. Vertebrates 

 are composed of segments placed one behind the other, as in 

 the Annulosa ; but the lines of junction have been more or 

 less effaced by structural modification, ;is, e.g., in the forma- 

 tion of the skull, which is composed of nine or more coalesced 

 segments. The threefold division of the body into head, 

 chest, and belly, so characteristic of shellfish and insects, is, 

 liowever, more obvious. Wherever limbs are pre-sent they 

 never exceed four in numlier, and are in pairs, whether as 

 fins of fish (not reckoning the unpaired fins as limbs), wings 

 and legs of biids and bats, fore and hind legs of quadrupeds, 

 or arms and legs of man ; all being modifications of one 

 type — as, for example, in the framework of the bat's wing, 

 the bones forming which, although immensely prolonged, 

 correspond to our fingers. 



Such, in crude outline, are the principal features of the 

 highest animals, but no general description can cover the 

 infinite variety of vertebral forms. Some, as the lancelet 

 and shark, have no bony spine ; the frog has no ribs ; the 

 tortoise is encased in a shield composed of the hardened 



skin of its back and belly ; and even in the marked division 

 of vertebra,tes into cold-blooded, embracing fish and i-eptiles, 

 and warm-blooded, embracing l)irds and mammals, excep- 

 tions occur in warm-blooded fish, as the tunny and the 

 bonito. But no differences in detail can obscure the fact 

 that vertebrates are all modifications of a common type, 

 the variations in structure being due to differences of 

 function determined by unlike modes of life. Obviously 

 the highest types are those in which each organ has only 

 one thing to do, since where one organ has many things to 

 do each is done less completely. Moreover, details obscure 

 relations, and since it is with the relation of all life-forms 

 that we are chiefiy concerned, we may pa.ss to further 

 evidence of connexion between the highest invertebrate and 

 the lowest animal of vertebrate character. In water, the 

 population of which far outnumbers that of the land, and 

 the elements of which make up the larger proportion of 

 every plant and animal, life had its beginnings; and it is in 

 water that we find this connecting link in the shape of a 

 semi-transparent creature not more than two inches in 

 length, known as the lancelet, from its lance-like shape, and 

 also as Amphioxus (Gr. amphi, both, and oxus, sharp), be- 

 cause both ends are nearly alike. The mouth of this head- 

 less, feeble-eyed animal has cilia for driving in the food- 

 carrying water, and opens into a -svide gullet through the 

 breathing slits, in which the water, after giving up its 

 oxygen to the colourless blood, pa-sses into the body-cavity, 

 and is expelled at the vent. There being no muscular 

 heart, the blood is circulated by contractions of the vessels. 



Now this boneless creature is classed among back-boned 

 animals because it has the primitive gristly rod, called the 

 notochord, from which the spine is developed in all true 

 vertebrates. Above this rod Hes the nervous system, com- 

 posed of a single cord, which bulges slightly as a primitive 

 brain near the mouth. In describing the sea-squirt, or 

 ascidian, reference was made to its short notochord and 

 single nerve-ganglion, which correspond, as far as they go, 

 to like organs in the lancelet ; and if they were lengthened 

 so as to run along the whole of the back of the ascidian, the 

 positions in the two animals would be found to agree exactly. 

 This certainly points to their common descent, and when we 

 compare the embryos and mode of development of both, the 

 evidence will be found complete. 



Fishes, as the least specialised vertebr-ates, are placed in 

 the lowest class, many species, as sharks, rays, and sturgeons, 

 representing in their gristly backbones, uneven tails, and 

 spiny or plated skins, the armoured ganoids of Silurian and 

 Devonian seas, which are the earliest known vertebrate 

 forms and mark the gradations between cartilage and bone 

 in structure. But the vaster number of fishes, perhaps 

 embracing gi'eater variety of form than any other class of 

 vertebrates, are teleost, or bony-skeletoned, not breathing — as 

 the worm-like lampreys, hags (in which gristly arch&s for 

 enclosing the nerves appear on the notochord), sharks, and 

 their allies — through holes or slits in the neck, but through 

 gills. 



Just as we had to retrace our steps in search of a link 

 between vertebrates and invertebrates, so we must again go 

 back a step or txvo to find the intermediate forms between 

 aquatics and amphibians. These forms are represented by 

 certain fishes called Dipnoi, or " double-breathers," because, 

 while they have gills for taking up the oxygen from the 

 water, they can also bre:T.the on land by means of the air- 

 bladder or sound, which thus discharges the functions of a 

 lung. Such are the mud-Jish of the Amazons and the 

 jeevine of Australia, both of which show tendency towards 

 modification of the paired fins into limbs, those of the mud- 

 fish being thong-like, and those of the jeevine jointed for 

 locomotion on land. Other fish, as eels and the climbing 



