Morphology

Thus any investigation of structure from a fundamental stand point necessarily involves a comparison with that of other ani mals, which may be held to be relatives in the sense that all have been derived by evolution from some common ancestor.

Such a comparison between two similar animals of different sizes will reveal a whole series of proportional differences which depend simply on absolute size. Thus an organ like the ear fulfils exactly the same function in a domesticated cat and in a lion perhaps fifty times as heavy. In each animal it has to determine rotational movements of the head, to determine its position with respect to gravity, and to receive sounds and analyse them into their constituent tones over a certain range. These things are done satisfactorily by the ear of a cat and even more so by those of smaller animals such as bats, and there is no reason for which the size of the organ should be increased however large the animal may become. In fact the ear of a lion is only about three times the volume of that of a cat. But generally the size of an organ necessarily varies with that of the animal of which it forms a part, and varies in accordance with mathematical laws. For example the weights of two similar animals will vary as the cube of their linear dimensions, while the areas are only related as the squares.

Thus if the larger of

two cats was a mere mechanical enlarge ment to twice the length of the other, its weight would be eight times as great, while the area of the transverse section of a leg bone would be only four times as large, the pressure tending to crush it being twice as great for a unit area. Thus if there is to be the same factor of safety in the two cases the leg-bone of the larger animal must be thicker, so that its diameter will be instead of V i.e., instead of twice the diameter, it should be nearly 24 times as large.

Modifications in accordance with this law apply to all the organs of the body, to the lungs and alimentary canal, to the brain and spinal nerves as well as to the skeleton. But after allowance has been made for all such differences, there will still remain others which cannot be accounted for on mechanical grounds but represent innate characters of the two animals.

If the comparison be carried further afield, as it must be if the nature of the general plan of the structure is to be determined, fresh difficulties arise.

A comparison of a bird with a mammal will show a similarity of external structure in that each is bilaterally symmetrical, has a head, neck, trunk and tail, and two pairs of symmetrical appen dages. But in the mammal the anterior pair are walking legs, in the bird they are wings. The problem which at once arises is whether it is justifiable to compare these structures with one another or whether they are sui generis.

This problem may be attacked in several ways which depend ing on different lines of reasoning should give consistent results.

Each appendage is covered with skin, which in the one case supports feathers, in the other fur, but as this skin is continuous with and of the same general character of that which coats the body it is clear that the differences between their qualities is not a special feature of the organ which we are discussing.

Below the skin in each case lies a series of bones and muscles, together with the blood vessels and nerves which are necessary for their maintenance and control.

In

each case the skeleton consists of an articulated series of bones forming functionally a series of interconnected levers. The segment lying nearest the body is single, articulating with a cup formed by two bones which lie within the body; the second seg ment contains two bones lying side by side which at their outer ends articulate with a series of small bones, the wrist, beyond which lie in one case the bones supporting five fingers, in the other a strange bone with a slit dividing it incompletely along its length, which in turn supports others forming three axes. Thus the proximal parts of the skeletons of the two appendages have a similar arrangement, while distally they differ considerably. The muscles, blood vessels and nerves show similar resemblances and differences.