Origin of Different Kinds of Adaptations 347 
When the young flounders leave the egg, they swim in an 
upright position, as do ordinary fishes, with both sides equally 
developed. There cannot be any doubt that the ancestors 
of these fish were bilaterally symmetrical. Therefore, within 
the group, both right-handed and left-handed forms have 
appeared. It seems to me highly improbable that if a right- 
handed form had been slowly evolved through the selec- 
tion of favorable variations in this direction, the end result 
could be suddenly reversed, and a perfect left-sided form 
appear. Moreover, as has been pointed out, the intermediate 
stages. would have been at a great disadvantage as compared 
with the parent, and this would lead to their extermination 
on the selection theory. If, however, we suppose that a vari- 
ation of this sort appeared at once, and was fixed, —a muta- 
tion in other words, —and that whether or not it had an ad- 
vantage over the parent form, it could still continue to exist, 
and propagate its kind, then we avoid the chief difficulty of 
the selection theory. . Moreover, we can imagine, at least, 
that if this variation appeared in the germ and was, in its 
essential nature, something like the relation seen in the snail, 
the occasional reversal of the relations of the parts presents 
no great difficulty. 
In this same connection may be mentioned a curious fact 
first discovered by Przibram and later confirmed by others. 
If the leg carrying the large claw of a crustacean be removed, 
then, at the next moult, the leg of the other side that had 
been the smaller first leg becomes the new big one; and the 
new leg that has regenerated from the place where the big 
one was cut off becomes the smaller one. 
Wilson has suggested that both claws in the young crusta- 
cean have the power to become either sort. We do not know 
what decides the matter in the adult, after the removal of one 
of the claws. Some slight difference may turn the balance 
one way or the other, so that the smaller claw grows into the 
larger one. At any rate, there is seen a latent power like 
