154 Francis B. Sumner 
for the entire lot of animals. The diminution of differences in 
weight between the contrasted groups of animals is less certain, 
though it appears tolerably clear in the case of the 1907-1908 
females. It will be recalled, however, that the differences in 
weight were only very doubtfully regarded as results of the tem- 
perature conditions. 
This tendency toward a reduction of experimentally produced 
differences in the relative size of parts, should it prove to be gen- 
eral, is of considerable theoretic interest. It adds another to the 
many well-known examples of a “regulative” tendency in living 
things.*° After the initial shock of change, with its resulting effect 
in deflecting the organism away from its individual norm, there 
‘would seem to be a continuous effort to regain the latter. Here 
we have a principle which might be said to bear the same relation 
to individual growth as Galton’s “law of filial regression” bears to 
stem-history, though the analogy may be merely superficial. In 
either case, however, we have to do witha “reversion to mediocrity.” 
The process in question is directly opposed to that conceived of 
by Weismann, in his theory of “germinal selection,” as occur- 
ring among the determinants of the germ-plasm. According to 
this hypothesis, a given determinant, if once handicapped by un- 
favorable nutrition, is more and more pushed to the wall by its 
more fortunate competitors until it may be totally annihilated. 
The disappearance of useless structures in phylogeny and the fre- 
quent orthogenetic trend of evolution is thus explained. If it 
be objected that this analogy of mine is out of place I can only 
reply that Weismann’s whole conception of a struggle among the 
determinants of the germ-plasm was derived from what was 
assumed to occur among the parts of the organism as a whole. 
Some evidence has been offered above for the existence of a tend- 
ency in the growing body quite at variance with the demands of 
that theory. ‘To many readers, on the other hand, it will doubt- 
% Vernon’s principle that ‘‘the permanent effect of environment on the growth of a developing organ- 
ism diminishes rapidly and regularly from the time of impregnation onwards” (op. cit., p. 199) would 
account for the failure of these differences to augment with the growth of the organism. But it cer- 
tainly would not in itself account for the absolutely greater increase shown by the more retarded organs 
(or organisms) mentioned on p. 137 above. 
