842 



LIFE : OUTLINES OF GENERAL BIOLOGY 



have had a great influence on the evolution of grazing mammals, 

 and it must also have meant a great reduction of superficial erosion. 



The fourth of the Tertiary periods, the Pliocene, seems to have 

 been a time of restless strenuous life, with many changes in the 

 surface features of the earth and with many trekkings and explora- 

 tions, as well as eliminations. It was probably in the Pliocene, that 

 tentative men began to appear, and there was probably much sifting 

 in their evolution. 



Pleistocene or Quaternary. — In the Cenozoic Era man was 

 still on trial, and not yet at the level of Homo. The elevation of the 

 continental masses went on and a combination of factors led to the 

 setting in of the prolonged Glacial Periods or Ice Ages, interrupted 



Fig. 144. 



Antlers of Stag, from second year onwards (1-5). The points A-D on the 

 six-year-old stag are called tynes, which are increased in number with 

 each fresh growth. The antlers fall off at the end of the season, and 

 the growth next year begins again at the beginning. The figure would 

 also serve in a general way to indicate the increase of complexity in the 

 course of evolution through successive geological periods. In other words, 

 the figure may serve for phylogeny as well as for ontogeny. 



by three milder Interglacial times. These times of smothering ice- 

 sheets and fatal cold involved no doubt terrible elimination, but they 

 also served in some cases to stimulate endeavour at many levels. 

 It was towards their close that Homo definitely emerged and was 

 well represented by Neanderthal Man. 



GREAT STEPS IN STRUCTURAL ADVANCE 



In thinking of Organic Evolution, one naturally concentrates 

 attention on the emergence of the great classes : When and whence 

 was the origin of Birds or Insects, Seed-Plants or Ferns, and so 

 forth ? But prior to these particular problems, there are larger ones 

 — the origin of the multicellular body, of sex, of blood, of brains, of 

 limbs, of viviparity, and so on. Even though one cannot as yet 

 advance far in explaining these and other great achievements of 



