Effects of External Conditions 153 
ties, even while the conditions which gave rise to them remain 
in full force. A diminution in initial differences of size has been 
demonstrated by Minot* in the case of growing guinea-pigs. His 
findings upon this point are thus summed up: “The study of the 
individual variations yields two important conclusions: Furst, 
that any irregularity in the growth of an individual tends to be 
followed by an opposite compensating irregularity. Second, the 
variability diminishes with the age.’’ Thus, “if an individual 
grows for a period excessively fast, there immediacely follows a 
period of slower growth, and vice versa, those that remain behind 
for a time, if they remain in good health, make up the loss (at least 
in great partif not alwayscompletely)soonafter. . . . . It 
is probable that the same is true for man and that therefore the 
usual and even the severer illnesses of childhood and youth do 
not greatly affect the ultimate size of the adult.’’ Pearson,* 
likewise, has shown that the variability both of weight and of 
stature in man diminishes from infancy to adult life. And indeed 
it is a matter of common experience that an early handicap in the 
size or strength of a child is frequently “outgrown,’’ wholly or in 
part. 
The variability which the above-named writers have considered 
is doubtless in part due to blastogenic differences, 1n part to somato- 
genic ones, resulting from fetal or post-natal conditions of nutrition, 
etc. In my own results, however, the most noteworthy fact is not 
a reduction in the general variability of my stock, but the diminu- 
tion of differences whose cause is knownto be external—and this 
while the effective conditions remain unchanged. ‘The foregoing 
statement applies to the growth of the tail, both of the male and the 
female mice, between the age of six weeks and the age of 24 (or 3) 
months. It likewise holds, with some qualification, for che growth 
of the tail during the next interval between the measurements, 1. e., 
between 24 and 7 months. In the latter case, however, the data 
are fewer, and the allowance is necessary that about midway dur- 
ing this third period the temperature conditions were equalized 
48 Senescence and Rejuvenation. Journal of Physiology, vol. xii, no, 2. 1891, PP- 97-153, pl. ii-iv. 
“8 Proceedings of the Royal Society, Ixvi, 1900, p. 23 (cited by Vernon, in ‘‘Variation in Animals 
and Plants”). 
