during the Formation of the Crust of the Earth. 87 



maHne omnivorous and carnivorous types^ and, amongst the 

 genera or the orders belonging to these classes, those which 

 feed upon Cryptogamia and Gymnospermia, were able to exist 

 at a more ancient period. Amongst the multitude of animals 

 which live at the expense of the angiospermous Dicotyledons, 

 there is, moreover, a great quantity which are dependent upon 

 one another ; thus the carnivorous Vertebrata^ the coprophagous 

 or parasitic Insects, kd, could only make their appearance after 

 certain other animals. 



15. Progressive development does not consist only in the fact 

 that new and more perfect types became added to the inferioi- 

 types which existed before, but also in the circumstance that 

 these latter decreased in importance from their point of cul- 

 mination, and finally became entirely extinct. As a matter of 

 course, certain types appeared from the first with their maximum. 

 We consequently find simultaneously in each subkingdom, and 

 even in each class of organized beings, types in course of develop- 

 ment and others in course of diminution. The types which have 

 •a tendency to disappear are inferior types in regard to their 

 organization or to the terripetal series (for example the Cepha- 

 lopoda). The types which go on multiplying, on the contrary, 

 occupy a higher place in one or other of these points of view. 

 The groups which tend thus to replace each other are generally 

 met with in the mesolithic period ; but sometimes also they are 

 separated by a longer or shorter interval. Besides, there exist 

 groups of organisms of which the numerical development re- 

 mains nearly the same through all periods. These are, for the 

 most part, inferior orders or suborders, composed, perhaps, some- 

 times of two groups tending to replace each other. 



16. All the great phsenomena relating to the order of ap- 

 pearance of the difi*erent subdivisions of the organized kingdom 

 result from the laws which we have here developed, and which 

 may be summed up as follows : — a, adaptation to external con- 

 ditions; bj terripetal movement; c, progressive development, 

 that is to say, the successive appearance of forms with more and 

 more complicated organization. The apparition of all these 

 subdivisions is subordinated to these laws, with the exception of 

 certain groups of secondary importance (suborders or families). 

 Among these few and unimportant exceptions we may cite the 

 late appearance of certain groups of Teleostian fishes, the pre- 

 mature appearance of certain terrestrial Reptiles (Thecodont and 

 Acrodont Lacertians) which preceded the aquatic Saurians 

 (Nexipoda and Emydosauria), and the rapid extinction of the 

 Binosauria, with so high an organization, at the moment of the 

 appearance of Mammalia*. But these facts are so isolated, that 



* As Dinosaurian reptiles occur in the Trias and the Lias, and as small 



