March 23, 1876] 



NA TURE 



411 



to the origin of the Anglo-Saxons by showing that they 

 came from Friesland in the sixth or seventh century. 

 The same remark applies to the origin of nearly all fishes, 

 in fact, only one clear case of progressive modifi- 

 cation is known in the whole class ; this is afforded by 

 the group of the Pycnodonta. 



These are fish not unlike our John Dory in shape, 

 which appear for the first time in the carboniferous 

 rocks, and become extinct in the older tertiaries ; they 

 are distinguished by the 'possession of rows of large 

 crushing teeth, and in place of a vertebral column had a 

 gelatinous chorda dorsalis or tiotochord. The spinal cord 

 above this was embraced by arches of bone, placed at 

 regular intervals along the chorda ; and, immediately 

 below these neural arches, were attached the ribs, also 

 bony. In the Carboniferous forms, both arches and ribs 

 are quite distinct from one another, and are simply united 

 by ligamentous fibres to the notochord ; but, in the older 

 Secondary species, they become expanded at their ends, 

 and thus tend to embrace the notochord ; and, lastly, in 

 the Tertiary pycnodonts this process is carried to such an 

 extent as almost to produce a ring of bone, like the body 

 of a rudimentary vertebra. 



Now let us turn to the next group of Vertebrate animals, 

 that of Amphibia (frogs, toads, newts, and salamanders), 

 which are distinguished from fishes by certain very 

 striking peculiarities. Fishes are all capable of breathing 

 the air dissolved in water by means of giUs, and — a far 

 more important distinctive character — their limbs always 

 have the character oi fins, which organs are seen in their 

 simplest form in Ceratodus. In this fish, there is a long 

 jointed cartilaginous axis, running down the middle of the 

 fin, with rows of rays of the same substance on each side 

 of it ; the whole is invested by a fold of the integument, 

 the margins of which are beset with horny filaments called 

 fin-rays. In all fishes these elements are to be found, 

 generally in a curiously modified condition ; in the bony 

 fishes, for instance, the central axis with its side appen- 

 dages are broadened out and shortened, the fin-rays 

 becoming at the same time so much larger as to form the 

 main part of the fin. 



Some modification of this type of limb is possessed by 

 all fishes which have limbs at all ; but the first character, 

 that afforded by the respiratory organs, is not absolute, 

 for there are some fishes which, besides gills, possess an 

 apparatus for breathing air directly. This apparatus, 

 represented by the air-bladder of ordinary fishes, first 

 takes on its new character and becomes a lung in that 

 remarkable genus, Ceratodus, in which it exists as a 

 large cellular structure situated in the upper part of the 

 abdominal cavity, just under the vertebral column, and 

 connected with the gullet by a slit — the glottis — by means 

 of which the fish can pass air from the mouth into the 

 lung. It is not, however, this peculiarity of opening into 

 the oesophagus which constitutes a lung, for the air- 

 bladder of many fishes possesses an open duct of a simi- 

 lar nature ; the great distinguishing feature is, that the 

 blood taken to this bladder does not pass into the ordi- 

 nary venous channels, but is returned immediately to the 

 heart, in a purified condition, by a special vein. In Cera- 

 todus there is no special vessel to carry blood to the lung, 

 in other words, although there is a pulmonary vein, the 

 pulmonary artery has not appeared ; but in the Mudfish 

 (Lepidosiren) of Africa and eastern South America, the 

 development of the lung goes a step farther, a special 

 pulmonary artery being present, as in all the higher 

 animals. Thus Ceratodus and Lepidosiren are truly 

 amphibious, for they can be suffocated neither by removal 

 from water like most fish, nor by immersion in water like 

 the higher animals. 



What constitutes the difference between these amphi- 

 bious fish, and the lowest of the true Amphibia ? Not 

 the nature of the respirator)' process, for many of the 

 latter group, such as the blind Proteus of the Austrian 



caves and the North American Menobranchus possess 

 gills throughout life, but the structure of the limbs, which 

 are now, no longer fitis, but le^^s. A fish requires a broad 

 surface for balancing itself in the water, locomotion being 

 chiefly performed by the tail, but in land animals an 

 apparatus is required capable of raising the body above 

 the ground, and the limbs take on the form of a set of 

 jointed levers. In its simplest form the higher vertebrate 

 limb consists, first, of a single piece of cartilage articulated 

 with the body, then two pieces side by side, then a num- 

 ber of small nodules, and lastly, five series of short jointed 

 pieces ; all of these become in the adult state more or 

 less converted into bone. The first or proximal division 

 of the limb is called the humerus in the fore limb, the 

 femur in the hind limb ; the next segment consists of 

 radius and ulna in the arm, tibia and fibula in the leg ; 

 the nodular pieces are respectively carpals and tarsals, and 

 the series of jointed bones or cartilages, the five digits. 

 From the lowest Amphibia upwards, the limbs, when pre- 

 sent, are always constructed upon this type. 



Nevertheless, the Amphibia still retain certain fish-like 

 characters, which are lost in the groups above them. They 

 all, at some period of life, breathe by means of gills, 

 although all have, in the adult state, lungs in addition. 

 Some forms, such as the Proteus and Menobranchus 

 mentioned above, retain their gills throughout life and are 

 hence called Perennibranchiates ; others, such as Meno- 

 poma, Amphiuma, &c., lose them in adult life, and are 

 called Cadncibranchiates. These two last genera, how- 

 ever, still retain traces of gill- clefts, but in all the Amphibia 

 with which we are acquainted in this country, the frog, 

 toad, and newt, even the clefts disappear, and the perfect 

 air-breathing character is assumed. 



These animals, in the course of their development, go 

 through a very singular series of metamorphoses, com- 

 parable to those by which a grub is converted into a 

 butterfly. At this season of the year, every pond is 

 almost certain to contain frog-spawn, masses of trans- 

 parent albuminous matter, with numberless imbedded 

 eggs, consisting of yolk, black on one side and white on 

 the other. A few hours after these eggs are laid, the 

 process of development begins by the formation of a 

 shallow groove, which appears quickly on the black, more 

 slowly on the white hemisphere, and is just such a 

 groove as would be produced by drawing a blunt instru- 

 ment along the equator of a soft globe. The egg is thus 

 divided mto two masses. A second form appears at right 

 angles to the first, dividing the whole o.'gg into four ; others 

 appear, in definite order, cutting it up into smaller and 

 smaller masses, until the whole yolk becomes granular, or 

 formed of microscopic cells. Two ridges then appear, on 

 the surface of the egg, and, uniting in the middle line, 

 inclose a cavity, the lining membrane of which is con- 

 verted into the brain and spinal cord. The head gradually 

 becomes differentiated, and the mouth appears on its 

 under side ; the tail grows out, and the little creature, 

 getting too long for the &%g, becomes coiled upon itself, 

 and, before long, ruptures the egg-membrane, and makes 

 its exit from its mass of jelly. 



It is now, to all intents and purposes, a fish ; it has no 

 limbs, its mouth is provided with horny jaws, and it 

 breathes by means of a pair of plumose giUs, It further 

 differs from the adult frog in being herbivorous, feeding 

 on water plants, to which it attaches itself by means of 

 two suckers near the mouth. The tadpole grows rapidly, 

 and, before long, a fold of skin appears on each side, 

 which gradually closes over the gills, leaving, however, 

 for a considerable time, a small opening on the left side. 

 In the meantime the limbs appear, and the lungs are 

 developed, the tadpole breathing for a time both by lungs 

 and gills ; the latter eventually disappear, the tail shortens, 

 the limbs lengthen, the homy jaws are replaced by teeth, 

 and an insect-eating tail-less frog is formed, the adult air- 

 breathing fonn having thus been attained by a wonderful 



