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. 2. From Apes back to lowest Lemurs in the lowest Eocene. The date of Eocene being fixed at 3,000,000, we have about 2,100,000 years for this stage; assuming as much as five years for puberty, this results in 420,000 generations. 3. From Lemures to Prototheria. The earliest known mammalian remains come from the Rhætic, or top formation of the Triassic epoch; allowing for the Rhætic only 100,000 years, we have to add the whole of the Jurassic and Cretaceous, in all about 5,500,000 years. Assuming three years for a generation, we get 1,800,000 generations. 4. From Prototheria to something like the Theromorpha at the bottom of the Triassic strata. A duration of 1,700,000 years divided by four gives 425,000 generations. 5. From Theromorpha to Proreptilia, represented by Eryops and Cricotus from the Lower Permian of Texas. Allowing 1,000,000 years, each generation at four years, we obtain 250,000 generations. 6. From Proreptilia to Eotetrapoda, the first terrestrial Vertebrata, represented by something like the Stegocephali, the earliest of which are known from the Coal-measures. Assuming them to have come into existence at the bottom of the Coal-measures, for the duration of which we may guess 2,000,000 years, we get, with four years' allowance for puberty, 500,000 generations.