1898] SPECIES, SEX, AND INDIVIDUAL 237 
and of fin-rays, and that the terrestrial form of limb, transversely 
jointed into three segments and divided at the extremity into five 
digits, was not evolved from the fin of a fish, but was a new organ. 
Such a view is very improbable and by no means inevitable. It is 
much more reasonable to suppose that the terrestrial limb was: 
evolved by the modification of the fin of a fish. The tadpole has 
lost its limbs, because in its short life their use has become 
diminished. It does not sustain itself in the water, but fixes itself 
to plants by means of its suckers, and moves from one place to 
another by violent strokes of its tail. Its habits have been almost 
as much altered as those of the frog, and its structure has been 
determined by its habits. Thus from an ancestral fish has been 
evolved a creature passing by a well marked metamorphosis from a 
larval aquatic stage to an adult terrestrial stage, and in each of 
these stages it has become very different from its ancestors. 
The original reptiles were derived from the Amphibia by a 
change in the character of their eggs, which acquired large yolks 
_ and were enclosed in tough shells. Within the shell ‘the larva was 
retained, never being set free in the water, and thus for the first 
time terrestrial vertebrates became entirely independent of a liquid 
medium. The embryo in the latter evolution of terrestrial verte- 
brates has undergone various modifications, but the condition in 
which we find it at the present day is the original larval condition 
of the amphibian ancestor, except so far as it has been modified by 
the conditions of development within the egg-shell or the uterus. 
Thus the course of development in the higher vertebrate is not to 
be explained by the law of recapitulation, according to which tran- 
sient embryonic stages represent ancestral structures, but by the 
fact that the embryonic stage is a larval stage which passes through 
its metamorphosis before hatching or birth. The larval stage and 
the metamorphosis were originally determined by the temporary 
conditions of life in the individual, and the persistence of larval 
characters in the embryo is due to the fact that there has been 
nothing in the conditions of embryonic life to change them. 
Let us turn now to another instance, namely, the transformation 
of the flat-fishes. Perhaps it will be thought that there can be no ex- 
euse for throwing doubt upon the accepted doctrine that the larva of 
these fish swimming upright in the water with an eye on each side 
of its head, repeats in individual development the conditions of the 
ancestor. But a more careful study of the facts shows that this 
doctrine is erroneous, or at least only a partial truth, and it must be 
modified to agree with the state of knowledge at the present time. 
A brief summary of the facts will be sufficient to prove this. 
The flounder when first hatched is a minute larva not quite 
3th in. in length. The right and left sides are perfectly similar to 
