In the acceleration-curves which we have shown above (Figs. 2, 3), it will be seen that the curve starts at a considerable interval from the actual date of birth; for the first two increments which we can as yet compare with one another are those attained during the first and second complete years of life. Now we can in many cases “interpolate” with safety _between_ known points upon a curve, but it is very much less safe, and is not very often justifiable (at least until we understand the physical principle involved, and its mathematical expression), to “extrapolate” beyond the limits of our observations. In short, we do not yet know whether our curve continued to ascend as we go backwards to the date of birth, or whether it may not have changed its direction, and descended, perhaps, to zero-value. In regard to length, or stature, however, we can obtain the requisite information from certain tables of Rüssow’s[103], who gives the stature of the infant month by month during the first year of its life, as follows: Age in months 0 1 2 3 4 5 6 7 8 9 10 11 12 Length in cm. (50) 54 58 60 62 64 65 66 67·5 68 69 70·5 72 [Differences (in cm.) 4 4 2 2 2 1 1 1·5 ·5 1 1·5 1·5] If we multiply these _monthly_ differences, or mean monthly velocities, by 12, to bring them into a form comparable with the {74} _annual_ velocities already represented on our acceleration-curves, we shall see that the one series of observations joins on very well with the other; and in short we see at once that our acceleration-curve rises steadily and rapidly as we pass back towards the date of birth. [Illustration: Fig. 7. Curve of growth (in length or stature) of child, before and after birth. (From His and Rüssow’s data.)] But birth itself, in the case of a viviparous animal, is but an unimportant epoch in the history of growth. It is an epoch whose relative date varies according to the particular animal: the foal and the lamb are born relatively later, that is to say when development has advanced much farther, than in the case of man; the kitten and the puppy are born earlier and therefore more helpless than we are; and the mouse comes into the world still earlier and more inchoate, so much so that even the little marsupial is scarcely more unformed and embryonic. In all these cases alike, we must, in order to study the curve of growth in its entirety, take full account of prenatal or intra-uterine growth. {75} According to His[104], the following are the mean lengths of the unborn human embryo, from month to month. Months 0 1 2 3 4 5 6 7 8 9 10 (Birth) Length in mm. 0 7·5 40 84 162 275 352 402 443 472 490–500 Increment per month in mm. — 7·5 32·5 44 78 113 77 50 41 29 18–28