How far the phenomenon is due to actual growth of the arms and how far to the increasing breadth of the chest is not yet ascertained.
The differences of rate of growth in different parts of the body are very simply brought out by the following table, which shews the relative growth of certain parts and organs of a young trout, at intervals of a few days during the period of most rapid development. It would not be difficult, from a picture of the little trout at any one of these stages, to draw its apôÙproxôÙiôÙmate form at any other, by the help of the numerical data here set forth46.
| Days old | Total length | Eye | Head | 1st dorsal | Ventral fin | 2nd dorsal | Tail-fin | Breadth of tail |
|---|---|---|---|---|---|---|---|---|
| ã49 | 100ãã | 100ãã | 100ãã | 100ããã | 100ããã | 100ãã | 100ãã | 100ãã |
| ã63 | 129ôñ9 | 129ôñ4 | 148ôñ3 | 148ôñ6ã | 148ôñ5ã | 108ôñ4 | 173ôñ8 | 155ôñ9 |
| ã77 | 154ôñ9 | 147ôñ3 | 189ôñ2 | (203ôñ6) | (193ôñ6) | 139ôñ2 | 257ôñ9 | 220ôñ4 |
| ã92 | 173ôñ4 | 179ôñ4 | 220ôñ0 | (193ôñ2) | (182ôñ1) | 154ôñ5 | 307ôñ6 | 272ôñ2 |
| 106 | 194ôñ6 | 192ôñ5 | 242ôñ5 | 173ôñ2ã | 165ôñ3ã | 173ôñ4 | 337ôñ3 | 287ôñ7 |
While it is inequality of growth in different directions that we can most easily comprehend as a phenomenon leading to gradual change of outward form, we shall see in another chapter47 that differences of rate at different parts of a longitudinal system, though always in the same direction, also lead to very