March 30, 1876] 



NA TURE 



429 



PROF. HUXLEY'S LECTURES ON THE EVI- 

 DENCE AS TO THE ORIGIN OF EXISTING 

 VERTEBRATE ANIMALS '' 



III. 



IT will be necessary to preface our remarks as to the 

 origin of the next highest group of Vertebrates — that 

 of Reptiles — by some account of the distinction between 

 them and the Amphibia, and by some observations on 

 what zoologists mean by the terms " higher" and "lower" 

 as applied to animals or groups of animals. 



In external form there is little difference between such 

 a reptile as a lizard, and such an amphibian as a newt, 

 and there seems, at first sight, to be no reason why they 

 should be placed in different primary groups. In former 

 times, as a matter of fact, the essential difference between 

 reptiles and amphibians was not seen, and the two were 

 united into a single class ; but modern researches have 

 shown that, beneath this external similarity, lie great and 

 important differences, the chief of which we must now 

 consider. 



In the first place, no reptile, at any period of its life, 

 possesses gills, and, in consequence, the breathing of air 

 dissolved in water becomes impossible. Nevertheless, 

 reptiles, in common with all the higher animals, have, at 

 one period in their existence, slits leading from the throat 

 to the exterior, in precisely the same position as the 

 branchial clefts of an amphibian, but functionless. 



Secondly, certain organs, known as " foetal appen- 

 dages," are developed in connection with the young 

 animal before it leaves the egg, and serve a temporary 

 purpose in its economy. In the possession of these 

 appendages, as well as in the absence of gills, reptiles 

 agree with birds and mammals, and differ from fishes and 

 amphibians. 



The young reptile is produced from an egg of relatively 

 large size, and consisting of a considerable mass of yolk, 

 surrounded by a quantity of transparent " white " or 

 albumen ; the whole being invested by a hard or soft 

 she'll. The yolk does not divide as a whole, but the pro- 

 cess of division is confined to a small patch on its surface ; 

 in fact, the reptilian egg answers to the amphibian egg, 

 plus a quantity of additional matter, called accessory, or 

 food*-yolk, which is unaffected by the process of yolk- 

 division. It is the small superficial patch, answering to 

 the whole amphibian egg, which is converted into the 

 body of the young reptile, the accessory yolk becoming 

 gradually smaller and smaller, as its substance is used up 

 ia the nourishment of the embryo ; in the meantime it 

 forms a bag attached to the umbilicus of the embryo, and 

 hence called the umbilical vesicle or yolk-sac \ it is the 

 first of the foetal appendages, and the only one which 

 occurs in any vertebrate below a reptile, being possessed 

 by certain fishes. 



After the embryo has attained a certain size, and has 

 come to lie, like an inverted boat, on the yolk-sac, a fold 

 grows up, all round it, from the surface of the yolk, and, 

 the edges of the fold coming together above, a bag is 

 formed enclosing the embryo into the interior of which a 

 watery fluid is secreted, in which the little creature lies. 

 This natural water-bed is called the amtiionj it is the 

 second of the foetal appendages, and no trace of it is to 

 be found in any fish or amphibian. 



The third and last of these curious embryonic appendages, 

 the allaniois, grows out from the tail-end of the embryo 

 as a pear-shaped body, solid at first, but soon converted 

 into a sac, which extends round the embryo and yolk-sac, 

 immediately beneath the membrane of the shell. The 

 cavity of the allantois acts as a receptacle for the nitro- 

 genous waste of the embrj-onic body, but its chief func- 

 tion is as a respiratory organ ; for this purpose it is 

 supplied by blood-vessels which form a close network 



' A course of six lectures to working men, delivered in the theatre of the 

 Royal School of Mines. Lecture III., March r3. Continued from p. 4x2. 



over its outer layer, and the blood contained in these 

 coming into close relation with the external air, through 

 the porous shell, readily exchanges its carbonic acid for 

 the atmospheric oxygen. 



As the embryo grows, the yolk-sac becomes smaller 

 and smaller, and is eventually completely dra%vn into the 

 interior of the body of the young reptile, which by this 

 time completely fills the shelL In many cases a homy 

 knob is developed on the nose, and, with this, the now ripe 

 embryo breaks the shell from the interior ; the amnion 

 and other membranes are burst, the allantoic circulation 

 is stopped, the first inspiration is taken, and the little 

 creature is born. 



There are several minor points in which reptiles are 

 distinguished from amphibia, amongst which we will only 

 mention the articulation of the skull to the first vertebra 

 by one condyle instead of two, the presence of a bone 

 called the basi-occipital in the hinder part of the skull 

 floor, and the fact that the branchial apparatus is reduced 

 in the adult to the small hyoid bone or cartilage, which 

 supports the tongue. 



In what respects is a reptile a higher organism than an 

 amphibian ? When one animal is said to be higher than 

 another, one of two things may be meant : its structure 

 may be more complicated, as a carved platter is higher 

 than a simple trencher ; or its parts may be so arranged 

 as to form a more complicated mechanism. The mere 

 repetition of parts does not raise an animal in the scale ; 

 a worm with a hundred segments is no higher than one 

 with ten, any more than a mill with ten pairs of stones is 

 a higher kind of machine than one with a single pair. 

 But if, instead of multiplying the number of millstones, 

 two pairs only were used, one of which was adapted for 

 coarse, the other for fine grinding, a machine of a far 

 higher order would be produced, and it is a similar dif- 

 ferentiation of parts for special uses and co-adaptation of 

 structures to given purposes which raises an animal above 

 its fellows. 



Judged by this standard, a reptile is a decidedly higher 

 animal than an amphibian ; its skeleton, for instance, is a 

 better piece of work, the joints being more neatly finished, 

 and the whole mechanism much more perfect. 



A third test is based on the facts of development. We 

 saw that a frog, in the course of its development, went 

 through a stage in which it was, to all intents and pur- 

 poses, a fish, and that it was only after passing through 

 this stage, as well as that of a branchiate amphibian, that 

 it attained its higher adult character. Now the reptile 

 stands in just the same relation to the amphibian, with 

 regard to its development, as the amphibian to the fish. 

 During the earlier stage of its growth it presents certain 

 amphibian characters, such as the presence of gill-clefts ; 

 but these lower stages are passed over ; the reptile goes 

 beyond the highest amphibian in its development, and is 

 therefore, in this respect also, to be considered as a higher 

 animal. 



At the present day there are four types of reptiles : the 

 lizards ■ {Laartilici), snakes {Ophidia), turtles and tor- 

 toises {Chelonid), and crocodiles {Crocodilia). W^e will 

 now direct our attention to the first of these groups. 



Most existing lizards have four well developed limbs, 

 a long tail, a scaly armour, sometimes supplemented with 

 plates of bone, and teeth, not set in distinct sockets, but 

 firmly fixed to the jaw. The skull is so constructed that 

 the hinder nostrils open far forwards into the mouth. 

 The vertebrae have a peculiar and characteristic form, 

 their articular surfaces being concave in front and convex 

 behind, except in the Geckos or wall-lizards, and that 

 remarkable New Zealand genus Hatteria or Sphenodon. 

 The heart is composed of three chambers, two auricles 

 and a single ventricle, the latter being again partly divided 

 into two, and thus showing a slight advance on the am- 

 phibian heart, in which the ventricle is quite single. 



Lizards are very abundant, especially in hot climates ; 



