In one of Quetelet’s tables (_supra_, p. 63), he gives measurements of the total span of the outstretched arms in man, from year to year, compared with the vertical stature. The two measurements are so nearly identical in actual magnitude that a direct comparison by means of curves becomes unsatisfactory; but I have reduced Quetelet’s data to percentages, and it will be seen from Fig. 19 that the percentage proportion of span to height undergoes a remarkable and steady change from birth to the age of twenty years; the man grows more rapidly in stretch of arms than he does in height, and the span which was less than {94} the stature at birth by about 1 per cent. exceeds it at the age of twenty by about 4 per cent. After the age of twenty, Quetelet’s data are few and irregular, but it is clear that the span goes on for a long while increasing in proportion to the stature. 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. [Illustration: Fig. 19. Ratio of stature in Man, to span of outstretched arms. (From Quetelet’s data.)] 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 approximate form at any other, by the help of the numerical data here set forth[126]. {95} _Trout (Salmo fario): proportionate growth of various organs._ (_From Jenkinson’s data._) Days Total 1st Ventral 2nd Breadth old length Eye Head dorsal fin dorsal Tail-fin of tail 49 100 100 100 100 100 100 100 100 63 129·9 129·4 148·3 148·6 148·5 108·4 173·8 155·9 77 154·9 147·3 189·2 (203·6) (193·6) 139·2 257·9 220·4 92 173·4 179·4 220·0 (193·2) (182·1) 154·5 307·6 272·2 106 194·6 192·5 242·5 173·2 165·3 173·4 337·3 287·7 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 chapter[127] that differences of rate at different parts of a longitudinal system, though always in the same direction, also lead to very notable and regular transformations. Of this phenomenon, the difference in rate of longitudinal growth between head and body is a simple case, and the difference which accompanies and results from it in the bodily form of the child and the man is easy to see. A like phenomenon has been studied in much greater detail in the case of plants, by Sachs and certain other botanists, after a method in use by Stephen Hales a hundred and fifty years before[128].