308 Explanation of Inheritance 
not prevented from passing at the proper moment of 
their development to terrestrial life. ‘One could say 
that this depends upon the amblystomal form being not 
yet sufficiently fixed in the heredity of the species, since 
the epigenesis resulting from this heredity does not yet 
necessarily cause the appearance of the amblystomal form 
so long as the conditions to which this form is adapted 
are not realized.” 227 
Finally the experiments of Cunningham on the colors 
of flat fishes already quoted above, are well known. He 
has shown that during their first metamorphoses, while 
the pigment is still present on both sides, the action of 
light artificially reflected upon the side of the fish which 
is turned toward the bottom does not prevent the pigment 
from disappearing even then from that side, so that in 
this case the color passes rapidly through a retrograde 
development. But a prolonged exposure to light pro- 
vokes the reappearance of the pigment on the lower side, 
and the pigment spots are in every respect like those 
which are normally present on the upper side of the 
fish.?28 
In this experiment then one has a clear and direct 
instance of a functional stimulus reinforcing the onto- 
genetic stimulus. We say “reinforcing” because the fact 
that the spots now appearing on the lower side are like 
those above, demonstrates that they are not produced 
de novo by the functional stimulus, but even now depend 
upon the ontogenetic stimulus, which, with the help of 
*°"Le Dantec: Traité de Biologie. P. 403-404. 
228Qsborn: The hereditary Mechanism and the Search for the 
unknown Factors of Evolution. Biol. Lect. at the Mar. Biol. Lab. of 
Wood’s Holl, Summer Session 1894. Boston, U. S. A., Ginn, 1896. 
P. or. 
