Alligators and crocodiles are known from the Upper Chalk; Tomistoma since the Miocene; Gavialis since the Pliocene.
The oldest surviving reptile is Sphenodon, the Hatteria of New Zealand, a fair representative of what generalized reptiles of the later Triassic period seem to have been like; and to the same period belongs Ceratodus, the Australian mud-fish, hitherto the oldest known surviving genus of a very ancient and low type so far as Vertebrata are concerned.
Now let us see if the above estimates of geological time are so utterly inapplicable to animal evolution. On purpose we take one of the lowest estimates, about 28,000,000 years, and apportion them equally to the various strata or epochs.
The original owner of the famous Trinil skull, a Pithecanthropus erectus, lived, according to some, in the Late Pliocene, according to others in the Early Plistocene, period—that is to say, somewhere about the beginning of our last Glacial epoch, some 270,000 years ago. Assuming that he and his like reached puberty at sixteen to twenty years of age, about 17,000 generations would lie between him and ourselves, or, to put it more forcibly, between him and the lowest living human races—say the Ceylonese Veddahs. Only 250 generations, at twenty years, carry us back to 3000 B.C. (i.e., beyond the ken of history); and if it be objected that the differences between the oldest inhabitants of Egypt, the Naquada, and the present Fellahin are very slight, we are welcome to multiply these differences sixty or seventy fold, in order to arrive at the Pithecanthropus level. But these Naquada had no metal implements, and there cannot be the slightest doubt that the development of the human race went on by leaps and bounds after certain discoveries had been made—to wit, the use of implements and that of fire. That creature which first took up a stone or a branch and wielded it thereby got such an enormous advantage over his fellow-creatures that his mental and bodily development went on apace. The same applies to the improvement of speech. We assume the single, monophyletic origin of mankind at one place, in one district; and the differences between some of the races of man are great enough to constitute what we might call species. Compare the Venus of Milo, that noble expression of the ancient Greeks' notion of female beauty, with the 'products of art' of the Veddahs or the dwarfs of Central Africa, or think of the beau-idéal which a Michael Angelo could possibly have evolved if he had never seen any but such people.
| I. | II. | III. | IV. | V. | VII. | VII. |
| Recent | } | Adam and Eve | 250 | |||
| Plistocene | } 5 | } | } | Man, contemp- | 3,500 | |
| } | } | } 270,000 | orary with Reindeer | |||
| } | } | in France | ||||
| Pliocene - | } | } 3,000,000 | Pithecanthropus | 16 | 17.000 | |
| } | } | } 600,000 | erectus | |||
| Miocene - | }10 | } | Anthropoid | 10 | 60,000 | |
| } | } | } 2,100,000 | Apes | |||
| Eocene - | } | } | Lemurs | 5 | 420,000 | |
| Cretaceous - | 10 | } | 3,600,000 | |||
| Jurassic - | 5 | } | 1,800,000 | |||
| Rhætic - | } | } | } | Prototheria, or | 3 | 1,800,000 |
| } | } | } | first Mammalia | |||
| } | } 7,200,000 | } | ||||
| Keuper - | } 5 | } | }1,800,000 | |||
| Muschelkalk - | } | } | } | |||
| New Red | } | } | } | Theomorpha | 4 | 425,000 |
| Sandstone | ||||||
| Magnesian | } | } | } | |||
| Limestone | } | } | } | |||
| Lower Red | } | } | } | Proreptilia | 4 | 250,000 |
| Sandstone | }15 | } | }4,000,000 | |||
| Coal-measures | } | } | } | Eotetrapoda | 4 | 500,000 |
| Mountain | } | } | } | |||
| Limestone | } | }17,500,000 | } | |||
| Devonian - | 15 | } | 4,000,000 | Dipnoi and | 5 | 1,000,000 |
| } | Crossopterygii | |||||
| Silurian - | 10 | } | 2,700,000 | First fishlike | 3 | 900,000 |
| } | } | creatures | ||||
| Ordovician - | 10 | } | 2,700,000 | |||
| Cambrian - | 15 | } | 4,000,000 | Sum total of | ||
| Laurentian - | generations | ———— | ||||
| Archean or | (about) | 5,375,000 | ||||
| Metamorphic |
Explanation of the Table on p. 149.
Column I. contains the names of the successive sedimentary strata.
" II. contains the percentage of the duration of the various epochs, according to Williams, the time from the Cambrian until recent times being taken as 100.
" III. gives the estimated duration in years of the Palæozoic, Mesozoic, and Cænozoic periods, according to Walcott.
" IV. gives in years the duration of the various smaller epochs, as computed from Walcott and Williams' statements.
" V. Representatives of stages of the ancestral line of man. The names stand in the level of the stratum in which they have made their first appearance.
" VI. contains the number of years which, in the present calculation, have been assumed necessary for the animal to reach puberty.
" VII. contains the number of generations which can have elapsed from stage to stage. For example, 60,000 generations separate the earliest known anthropoid apes from Pithecanthropus.
Let us follow the descent of man further back. The next stage, reckoning backwards, is that from Pithecanthropus to bonâ-fide anthropoid apes. They are represented in the Miocene by various genera—e.g., Pliopithecus and Dryopithecus. According to Croll and Wallace, 850,000 years ago carry us into the Miocene epoch. Assuming that these apes lived about 600,000 years before Pithecanthropus, namely, in the later half of the Miocene, and taking puberty at ten years of age, a high estimate, we get not less than 60,000 generations.