The Evolution of Limbs.

Oamaru Mail, Volume XXXV, Issue 10016, 5 December 1908, Page 3 (Supplement)

The Evolution of Limbs.

The original ancestor of all the vertebrates, different enough now as are the fishes, reptiles, amphiuians, birds, and beasts, was a little worm-like creature, with no backbone, no head separate from the body, and made up of repeated parts, ii.-.e a centipede '. (Tnat is what people who like hard words call metamerie reauplication.) The claim of the evolutionist, therefore, is that he can trace the slow development of the characteristic limbs of the "vertebrate from something that- happened in this limbless ancestor right down to that "periect instrument'' the nana, as elder writers used to call it, or that quite inaxoshui instrument the foot, originally meant for climbing—about which they never said anything. To trace the evolution of a limb tnrough the millions of years of a race nistory is, of course, immensely difficult, but it- cannot be impossible if evolution is a fact. And so for twenty or thirty years science has brought all its resources to bear on this problem. Bit by bit it has been able to reconstruct sections of the history, not in actual sequence, however, such as the growth of a bird's wing or a horse's foot, and now, though much remains to be clone, when it is all put together we can get a very complete picture of wnat has happened". The methods used to solve the problem are very various. One consists in. learning to recognise that limbs that look very dinerent in structure are fundamentally

the same. If we compare a man's hand with a bird's wing (the bony framework of each) we shall not find it very hard to see that there are the same bones in each, though their development has proceeded differently. Then the study of fossil forms has very fortunately brought to light the most wonderful of "'missing links'' m limb evolution, so that, for instance, we can trace every stage of the change of a five-toed plantigrade foot into the one-tced digitigrade foot of the horse—and when we see . the rocks yielding up such cinematograph 'pictures of the past it- gives us immense confidence not merely in evolution, but also in other lines of evidence to which perhaps we cannot apply so direct a test. This is the 'case, for example, with the evidence from embryology. Every animal begins its life as a single "cell, just like (on a superficial view) the ancestor of all animals and plants. As it. prows and changes we see it- pass through stages which in a general wav'correspond to tho whole line of race-evolution -that had preceded the actual adult form that is coming. We have in fact in each individual a kind of shorthand summary of the'history of the race from its' first beginnings—but it is rather illegibly written, and here and there amid the shock's and jars of time a page of it has been lost.' But enough re-

mains to throw a very wonderful light on the course of evolution. —The Beginnings of Limbs.— So we have to look foi- the beginnings of limbs in something that- might happen to a sort of limbless worm that lived in the water. If we look at the'simplest of vertebrates, the little Amphioxiis, we see folds of skin running continuously down its sides. If we watch the development- of a higher fish, its "fins," however strange they are in the adult, arise as a ridge in the skin, and as development goes on we shall see this rid"— perhaps," persist in a continuous line along the body, but mora often some parts will stop growing at an early stage, • and be lost*'in the growing body, while others will go ahead and form the variously shapediand placed fins that we know. But these nave bones in. them ? Well, queer as it looks, we do know as a matter of fact, that- the skeleton grew out of the skin. The first animal that ever had a skeleton had a skeleton outside Uts skin instead of inside, just as oysters and turtles, and things like that dp now. This exoskeleton is thus phylogenetically (we must get used to one or two hard names, it means in the history of the race), the older and conies long before the inside or endoskeleton. Embryology, in fact, shows quite clearly that our skeletons still start in our skins, and, to put it very crudely, work their way inwards. Fins, then, are lateral skin folds, originally i'u metameric reduplication, but now suffering reduction at ail points except where the "'fin"' remans. They may be paired or unpaired, be bony or not so bony, connected with the inside skeleton or not —it all depends on the stage of evolution the particular fish has reached. Median (unpaired) fins crop up again in the tadpole stage of amphibia, and in the newt they develop specially in the breeding season. Also we have fossil reptiles that possess these fins; but probably like the whales, these had gone back to the fish type as a result .of their aquatic habits;. their fin limbs are a return to, and not a relic of, the dim past. —From Fan to Limb.— The lateral fin-fold which gives rise to the paired fins of fishes is the source of all subsequent- limbs. The lateral fin theory is not altogether easy to follow out, but it has been .very thoroughly tested, and it holds its ground. Corresponding to the paired fins we see the gradual development- in the embryo, or evolution in the race, of the bony shoulder girdle and pelvis which are to support the fore and hind limbs respectively, and give them connection with the inside skeleton. In the fishes the pelvis remains much more simple and primitive than the shoulder. Though the limbs arise metamerically, they are not connected with any defined body segment; but- in different ■genera they vary greatly as to their relative position. In the earlier fishes the "shoulder 7 ' (pectoral arch) appears as a. eimpl'3 bar of cartilage, and this, condition is in embryology. Later on, bony structures arising from the skin develop, and more or less completely replace the primitive cartilage. The fin itself is connected with the arch by concave facets that fit into convex articulations. Li the higher fishes out of this simplest material of the pectoral arch a large clavicle develops. There is rather a gap between the fishes and amphibians, wrUnli Jll.-cn.lv ligvo +.!,» +.V.-.B 'nf sboil'WpT

; found in all the higher vertebrates. : the homology may be made : put. The reptiles show in their d'eye'lopL ment a plain connection with the shoulder ; gii'dle of fishes, and a. curious point is I that this is shifted further back from the head than in amphibia; as in these i again it is further back than in the fishes. , The bird wiiig support is the modified rep- - file-shoulder : (though in flightless ' birds . this has been more or less completely , reduced). The Monotremes have a very . primitive shoulder. The arch supporting - the hind limbs has a similar history. In - snakelike lizards and in snakes it nearly ' disappears, since there are no limbs to require it. But we see it in the embryo, and vestiges .remain in the adult. The . "marsupial bones" of monotremes and marsupials are a survival of structures in lower vertebrates, which .have been rei tained and developed to support the mar- , supial pouch. It is easy to see tliat all fish fins are formed on a single type, and therefore have developed from a' single i primitive form. It is certain that the fin. practically a single-jointed lever, was transformed into the many jointed level necessary for locomotion oil land, but there, is not much evidence as to the actual course of -development. —The Limb Up to Date.— '■ . There is no need to stress the point that 1 the limbs, of all the vertebrates above - fishes belong to a common type and have - had a common origin. Bone for bone, the reader with a book of comparative anatomy before him, can compare one with another. Whatever vertebrate he looks at he will find upper arm, forearm, wrist, hand; or thigh, shank, ankle, foot. Upper arm and thigh will always be one bone each; forearm and shank will always have two bones each. Fiiigers and toes will bs never greater than' five, and" some of them ' may have been lost' or strangely fused together. Never more than five—yet now and again, he will see a curious little bone ' that is probably the remains of a sixth we all" have lost. The" relative size of the bones, of course, may hia'ke the thing ; look altogether different in one form oi another; as in the old flying reptiles the fifth fiiiger is extended into a huge joined rod, but it is a. fifth finger all the same. The bird hand and arm are extraordinarily ' modified, but they are still the reptile forelimb.' and that old' Archaeopteryx had reptile claws on his toes. A bat's forearm has become a pseudo-wing, and if yon want the hind limbs of .a whale you will find only a few useless little bones. The five toes 'become one m the horse, an<! the embryo shows us the gradual reduc i t'ion taking place. Similarly camels ant i cows, pigs and sheep, and so on, have : kept two toes—we can watch them losim the other three. A sidelight on this Ijml

evolution is given by the occasional ap pearance of "extra" limb structures which are generally to be looked on at a chance recovery of "lost parts." 0: course, if a calf is borni with an extra lag ;this does not mean that calves ones •had five legs," but the explanation- of sucl freaks' as another, story.