INTRODUCTION 21 



of life their use has been abandoned. 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 meta- 

 morphosis from a larval aquatic stage to an adult terrestrial 

 stage, and in each of those 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 later evolution of terrestrial vertebrates 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 vertebrates is not to be explained by the law of 

 recapitulation, according to which transient 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 certain 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 trans- 

 formation of the flat-fishes. Perhaps it will be thought that 



