comparative osteology can have any conception of the number of experiments which were tried in different groups before the definite mode of forming a bony vertebra was attained.  At the same time the skull was developing in a somewhat similar manner.  But the skull is far more complex in origin and undergoes far more numerous and important changes than the simpler vertebral column.  Into its history we have no time to enter.

And what shall we say of bone itself as a mere material or tissue, with its admirable lightness, compactness, and flawlessness.  And every bone in our body is a triumph of engineering architecture.  No engineer could better recognize the direction of strain and stress, and arrange his rods and columns, arches and buttresses, to suitably meet them, than these problems are solved in the long bone of our thigh.  And they must be lengthened while the child is leaping upon them.  An engineer is justly proud if he can rebuild or lengthen a bridge without delaying the passage of a single train.  But what would he say if you asked him to rebuild a locomotive, while it was running even twenty miles an hour?  And yet a similar problem had to be solved in our bodies.

But the vertebral column is not perfected by fish.  The vertebrae with few exceptions are hollow in front and behind, biconcave; and between each two vertebrae there is a large cavity still occupied by the notochord.  Thus these vertebrae join one another by their edges, like two shallow wine-glasses placed rim to rim.  Only gradually is the notochord crowded out so that the vertebrae join by their whole adjacent surfaces.  Even in highest forms, for the sake of mobility, they are united by washer-like disks of cartilage.  Biconcave vertebrae persisted through the oldest amphibia, reptiles, and birds.  But finally a firm backbone and skull were attained.

2.  The appendages.  Of these we can say but little.  The fish has oar-like fins, attached to the body by a joint, but themselves unjointed.  By the amphibia legs, with the same regions as our own and with five toes, have already appeared.  The development of the leg out of the fin is one of the most difficult and least understood problems of vertebrate comparative anatomy.  The legs are at first weak and scarcely capable of supporting the body.  Only gradually do they strengthen into the fore- and hind-legs of mammals, or into the legs and wings of birds and old flying reptiles.

3.  Changes in the circulatory and respiratory systems.  The fish lives altogether in the water and breathes by gills, but the dipnoi among fishes breathes by lungs as well as gills.  As long as respiration takes place by gills alone, the circulation is simple; the blood flows from the heart to the gills, and thence directly all over the body; the oxygenated blood from the gills does not return directly to the heart.  But the blood from the lungs does return to the heart; and there at first mixes in the ventricle with the impure blood which has returned from the rest of the body.  Gradually a partition arises in the ventricle, dividing it into a right and left half.  Thus the two circulations of the venous blood to the lungs, and of the oxygenated blood over the body, are more and more separated until, in higher reptiles, they become entirely distinct.