Le Conth.] 



Critical Periods. 



329 



accompanying figure, in which the abscissa A B represents the 

 course of time, the ordinates the degrees of cold, and the height of 

 the line D E represents such a degree of cold as is necessary to 

 produce glacial conditions at sea level in middle latitudes. The 



D 















3 



4 



B 



gradual rise of the line of cold, A C, represents the gradual cooling 

 of the earth surface, whether by secular cooling or by decrease of 

 carbonic acid and water in the atmosphere, or by both; and the 

 waving of the line, the oscillations of temperature. The numerals 

 1,2, 3, 4, are the times of revolution or critical periods marked 

 by greater and more general oscillations, the smaller intervening- 

 waves being the more local oscillations. It is seen that only once, 

 i. e., during the last critical period, partly by increased general 

 refrigeration, but mainly by a concurrence of causes, geographical 

 and astronomical, true glacial conditions (represented by the line 

 D E) were reached. 



Critical periods, as already said, are of long duration and changes 

 during such times are not simultaneous everywhere, but rather prop- 

 agated from place to place. This is especially true of the changes 

 in organic forms. These are propagated by a wave of migration 

 which may extend far beyond the limits of the physical changes. 

 Therefore the time of greatest change in organic forms may not be 

 exactly synchronous everywhere. This fact gives sufficient room 

 for the application of Homotaxis. 



GENERAL FORMAL LAWS OF THE EVOLUTION OF THE ORGANIC 



KINGDOM. 



We may now profitably sum up the general formal laws, or the 

 general process, of the evolution of organic forms; for it is these 

 more than aught else which determine the divisions of time, both 

 primary and secondary. 



General Advance by all Factors. — Let us imagine then, first, a 



