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.