314 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



the primitive facoids resemble more those of the tropical zone, \rhile 

 those of the late periods for the most part approach in characteristics 

 the fucoids of the temperate zones, a fact which may serve as an indi- 

 cation in the determination of geological temperatures and climates. 



From the appearance of these first organisms until that of mau, life has 

 been manifested under every imaginable form in the varied conditions 

 of the different geological periods, for the biological evolutions have 

 been affected by certain causes which act with increasiug i)rogressiou 

 in immediate dependence upon the development of the earth's crust. 



It is to imlcontology we should apply for knowledge concerning the 

 development and succession of the animals and plants of past ages. 

 She employs the means placed at her disposal by geology and compara- 

 tive anatomy to reveal in each period and in each formation a world 

 whose strange forms have disappeared never to return. Each forma- 

 tion, however, does not represent an entire act of creation, but only a 

 disconnected page from the scene of this great drama,* consequently the 

 organic remains do not indicate to us the continuous succession of 

 organisms, but rather certain periods of their development, certain epi- 

 sodes in the history of creation. 



And what difficulties we encounter at every word in deciphering these 

 pages. We require a Cesalpin, a Leonardo de Vinci, a Palissy, and 

 a Bufifon to teach us their characters, and a Cuvier and an Owen to 

 show us how to read them. How many prejudices had to be overcome 

 before the learned men of the last century could be persuaded that these 

 singular forms found in the earth were other than simple freaks of 

 nature or the shells left by pilgrims upon the mountain-tops. The truth 

 at last was brought to light and prevailed over the misconceptions of 

 the world, but only very slowly were the facts obtained which allowed 

 any certain knowledge of organic remains, and a deduction of the sim- 

 ple laws which control their development. 



From analysis of these organisms and comparison of them with those 

 that are recent, we find that their differences of structure are greater, 

 in proportion as the strata which contain them are older ; also that in 

 the very ancient formations, the forms of high organiztion are more or 

 less wanting in ratio to the age of the rocks. Thus the mammals in the 

 Keuper (the upper division of the Triassic formation), the birds in the 

 Mixed Sandstone, the reptiles in the Old Red Sandstone, the fish in the 

 Upper Gray wacke. We may therefore conclude that at a certain period 

 there were no organisms whatever in existence. We might easily deter- 

 mine upon a certain period for the appearance of organisms, perhaps 

 that of the complete precipitation of the waters, but at present it con- 

 cerns us to know how they appeared, whether through the pre-existence 

 of germs or by spontaneous generation. This is a question of very 

 great importance, but concerning it we have still very little positive 



* Darwin, Geologische Jvfeinanderfolge organischer Wtsen, Zeitscbrift fur die <?csaium- 

 ten Naturwissenschaften, t. xvi, 1860, p. 428. 



