1 6 ANCESTRAL MAN. 



Eocene age ; those of the Meiocene age are assigned to the period 

 which occurred 850,000 years ago ; while the severe glaciation of 

 the Pleiocene age occurred within the limits of a period which 

 began 240,000 years ago, lasted for 160,000 years, and came to 

 an end about 80,000 years ago. At present we are receding from 

 glacial conditions ; the earth's eccentricity is diminishing, and 

 will continue to decrease for the next 23,900 years. [See Plate.'] 



IV.— FLINT- WORKERS. 



We are now in a position to consider the way in which the 

 various flint-workers were related to the last glaciation of 

 Europe. 



Between recurring glacial periods climate grows equable and 

 temperate, and remains so during a great lapse of time. It is 

 then that animal and vegetable species become fixed, and that 

 fixed species grow more and more specialised as their equilibrium 

 with their environment is established. It is true that individual 

 variation still goes on. Even to-day the amount of measurable 

 variation reaches the extent of from 10 to 20 percent, of the 

 average dimensions, and such variations occur not only in one 

 part of the body, they take place concomitantly in all parts. But 

 when external conditions do not change, or change very slowly, 

 these individuals derive no benefit from their variation. On the 

 contrary, they are steadily weeded out by the operation of the law 

 that organisms not in equilibrium with their environment must 

 perish. 



But during a glacial epoch, when alternations from a cold to 

 a warm climate happen every 10,000 or 12,000 years, individuals 

 capable of varying would have favourable variations preserved, 

 and new species would arise ; while species whose peculiarities 

 had become fixed, or aberrant, or extreme, ■ would be very liable 

 to destruction. 



At the middle, then, and at the close of any glacial epoch we 

 must expect to find two things, — the disappearance of some 

 animals and the advent of others. 



This will assuredly have been the case with the precursors of 

 man. When the Eocene cold had passed away ; while Britain 

 was still connected by dry land with America, Asia, and Africa ; 

 these ancestral forms left their warm arboreal retreats and spread 

 over the western plains of Europe. 



