1398] 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 lias 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 fiat-fishes. Perhaps it will be thought that there can be no ex- 

 cuse 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 

 £th in. in length. The right and left sides are perfectly similar to 



