1 8 THE EVOLUTION OF MAK. 



assigned to the relative durations of the five main divisions 

 or Epochs, the latter will be found to be about as follows : — 



I. Arohilithio, or archizoio (primordial) Epoch • • 68.6 



IL Palaeolithic, or palaeozoic (primary) Epoch , • 82^ 



HI. Mesolithic, or mesozoio (secondary) Epoch • • 11.6 



IT. Oasnolithic, or cenozoio (tertiary) Epoch . . • L8 



T. Attthropolithio, or anthropczoio (quaternary) Epoch . OJi 



Total ... 100.0 



The relative durations of the five main epochs of the 

 earth's- organic history, are yet more clearly seen in the 

 opposite Table (XIY.), in which the relative thicknesses of - 

 the strata systems deposited within these Epochs is repre- 1 

 sented on a scale corresponding to their actual depths. 



This table shows that the period of the so-called History 

 of the World forms but an inconsiderable span in comparison I 

 with the immeasurable duration of those earlier epochs during 

 which Man did not exist upon this planet. Even the great 

 Csenozoic Epoch, the so-called Tertiary Epoch, during which 

 the Placental Animals, the higher Mammals, developed, 

 includes but little more than two per cent, of the whole 

 enormous duration of the organic history of the world.^^ 



And now before we turn to our proper phylogenetic 

 task ; before, guided by our knowledge of ontogenetic facts 

 and by the fundamental law of Biogeny, we attempt to 

 trace step by step the history of the palseontological evo- 

 lution of our animal ancestors, let us turn aside for a short 

 time into another and apparently very different and very 

 remote department of science, a general review of which 

 will make the solution of the difficult problems which 

 now rise belbf« na verj m«ch easier. Tke science k thai 



