366 



THE CHANGING GENERATIONS 



we can best answer these questions by considering the genetic implications 

 of the observed facts. 



We can trace the ancestry of the frogs back as far as the fishes, with 

 some certainty; so let us begin there. Some hundreds of millions of 

 generations ago there were no frogs or other amphibians but only various 

 kinds of fishes. Some of these fishes, however, were the ancestors of the 

 first amphibians, and from some of these amphibians came the frogs. 



The ancestral fishes reproduced their kind according to the laws of 

 heredity. They possessed a particular gene complex, which determined not 

 only their characteristics as chordates, as vertebrates, and as fishes but 



Fig. 24.8. "Ontogeny recapitulates phylogeny." Individual and racial history of the key- 

 hole brachiopod. A to D, adult shells of species from four successive geological horizons in 

 the Jurassic and early Cretaceous periods. E to H, successive developmental stages in the 

 growth of a single Cretaceous species (Pygope diphyoides) . E, young, F, somewhat older. G, 

 more than half grown. H, adult. These growth stages correspond to the phylogenetic 

 history as shown in A to D. (Redrawn by permission from Schuchert and Dunbar, Outlines of 

 Historical Geology, 4th ed., published by John Wiley & Sons, Inc., 1941.) 



also just what kind of fishes they were. This original gene complex was 

 the basis of that now possessed by the frogs. Over great periods of time 

 it was slowly modified by mutation, recombination, and selection. The 

 descendants of the fish ancestors split into more and more kinds, each 

 with a different set of modifications of the old hereditary complex. Today 

 some of them are the frogs. The gene complex of these frogs is merely 

 that of the ancestral fishes plus all the modifications that have taken place 

 in it down through the endless generations. If the frog passes through 

 fishlike stages in its development, it is simply because it has fish genes 

 inherited from its ancestors. The same is true of the other resemblances 

 its embryo shows. The fact that it fails to become a fish and instead goes 

 through additional embryonic stages that make it a frog, is because of 



