WHAT IS EVOLUTION ? 725 



the highest structure only in the present time.* This series we will 

 call the geological or phylogenic series.f According to the evolution 

 theory, the terms of this series also are genetically connected. It is, 

 therefore, an evolution series. Furthermore, it is the most fundamen- 

 tal of the three series, because it is the cause of the other two. The 

 Ontogenic series is like it because it is a brief recapitulation, through 

 heredity, as it were from memory, of its main points. The Taxonomic 

 series is like it because the rate of advance along different lines was dif- 

 ferent in every degree, and therefore every stage of the advance is 

 still represented in a general way among existing forms. Some of 

 these points will be explained more fully in connection with the evi- 

 dences of the truth of evolution. 



It will be admitted, then, that we find progressive change in organic 

 forms throughout geological times. This is the first point in the defi- 

 nition of evolution. 



II. We have shown continuously progressive change in organic 

 forms during the whole geologic history of the earth, similar in a gen- 

 eral way to that observed in embryonic development. We wish now 

 to show that the laws of change are similar in the two cases. What, 

 then, are the laws of succession of organic forms in geologic times ? 

 I have been accustomed to formulate them thus : a. The law of differ- 

 entiation ; b. The law of progress of the whole ; c. The law of cyclical 

 movement.^ We will take up these and explain them successively, 

 and then, afterward, show that they are also the laws of embryonic 

 development, and therefore the laws of evolution : 



a. It is a most significant fact, to which attention was first strong- 

 ly directed by Louis Agassiz, that the earliest representatives of any 

 group, whether class, order, or family, were not what we would now 

 call typical representatives of that group ; but, on the contrary, they 

 were, in a wonderful degree, connecting links ; that is, that along with 

 their distinctive classic, ordinal, or family characters they possessed 

 also other characters which connected them closely with other classes, 

 orders, or families, now widely distinct, without connecting links or 

 intermediate forms. For example : The earliest vertebrates were 

 fishes, but not typical fishes. On the contrary, they were fishes so 

 closely conected by many characters with amphibian reptiles, that we 

 hardly know whether to call some of them reptilian fishes, or fish-like 

 reptiles. From these, as from a common vertebrate stem, were after- 

 ward separated, by slow changes from generation to generation, in 

 two directions, the typical fishes and the true reptiles. So, also, to 

 take another example, the first birds were far different from typical 



* This statement is general ; it will be modified hereafter. 



f Phute-gennao (kind-making) ; genesis of the race. 



\ This formulation of the laws of organic succession was given by me in 1860, before 

 I knew anything of either Darwin's or Spencer's evolution. They were my own mode of 

 formulating Agassiz's views. 



