36 LECTURES 



faculties of childhood, viz., perception and memory, 

 decline and become subordinate to the higher faculty 

 of imagination, and this, in turn, becomes subordinate 

 to the still higher faculty of productive thought; and 

 thus the whole organism becomes higher and more com- 

 plex. Each stage of development including not only its 

 own characteristic, but also, in a subordinate degree, 

 those of all preceding stages." 



D Precisely the same principles apply to social develop- 

 ment, where the forces characteristic of the previous 

 stages of its history are successively absorbed and included 

 by each succeeding stage. And, as development proceeds, 

 the entire social structure ever becomes more complex, 

 richer in detail, and occupies higher and higher planes. 

 All this is repeated in geologic history. 



Now in geology there are two methods employed for 

 determining the extent of its eras, ages, periods and other 

 divisions. We may, in the first instance, rely upon the 

 unconformitv of the rock-system, and in the second in 

 the change of the life-system. Almost without exception 

 the life-system is found to correspond with the uncon- 

 formity of the rock-system, and in the few cases where it 

 does not it should be followed as the safer guide. 



All over the world the great geologic eras, ages and 

 periods are the same, though the minor divisions of the 

 latter are non-cotemporaneous they are, nevertheless, 

 likewise similar. 



First, then, we have the grand divisions of the earth's 

 geologic history as a whole. These are designated as the 

 eras and there are five of them. The oldest of all is the 

 Arcnaean or Eozoic, which includes the Laurentian sys- 

 tem; above this occurs the Palaeozoic era, embodying the 

 Palaeozoic or Primary system; superimposed upon tnis is 

 the Mesozoic era, the record of which is embodied in the 

 Secondary system this is followed by the Cenozoic era, 

 whose history has been written in the Tertiary and Quar- 

 ternary systems; fifthly and lastly occurs the Psychozoic 

 era, which may well be designated as the era of mind, 

 it being the recent system or the system of the present 

 time. 



So again, as has already been intimated, the history of 

 the earth as a whole is susceptible of being divided into 

 seven ages, based upon the culminations during geologic 

 time of certain great groups of animals. These are, first, 

 the Archaean age which is measured by the rocks of the 

 Laurentian system; second, the age of mollusks, or the 

 age of invertebrates, including the Silurian formation of 



