168 Kepoet of the State GEOLoaiST. 



The examples of these persistent forms, which have remained 

 unchanged from the Cambrian, lessen the value of the argument 

 in quei^tion. [See foot-note.] In reality any form may sometimes 

 carry within itself some source of weakness ; it may be doomed to 

 disappear soon or late, conquered in the struggle for existence ; but 

 this fatality seems to depend in each particular case on special 

 causes, often discoverable, and not to an irresistible law, an 

 universal fatality which embraces all the individuals beyond 

 a certain organic level. 



Finally we will add, that so far as species are concerned, the 

 problem seems of little interest. If a form undergoes a rapid 

 transformation, are we justified in saying that it dies ? On the 

 contrary is not the process itself the very condition of life ? 



Law of improvement. — A more exact idea is obtained by a 

 simultaneous examination of the order of appearance and of the 

 degree of organic elevation of the leading types in the two 

 kingdoms — animal and vegetable. From this examination there 

 results, at first sight, a fact which has made a strong impression 

 on naturalists of every era; organisms have been constantly 

 improving from the first periods in which they are found in a 

 fossil state. 



This general law finds immediate application when we consider 

 the order of appearance of the large groups of the animal king- 

 dom. In the Cambrian are found Sponges, Cystideans, Brachio- 

 pods. Worms, G-asteropods, Crustaceans. In the Ordovician 

 appear the Crinoids; in the Bohemian [Silurian], Arachnids, 

 Insects, Fishes ; the Batrachians, not yet of high degree, are 

 found in the Devonian ; and the Keptiles, still represented by the 

 lower forms of the group, in the Carboniferous, l^ot until we 

 arrive at the Trias do we find the first Mammals, and no birds 

 appear before the Upper Jurassic. 



The first Mammals are all Marsupials, and it is only in the 

 Eocene epoch that the first Placentals appear. If we consider 

 a smaller group, for instance, the Cephalopoda, we see that the 

 Tetrabranchs precede the Dibranchs ; the succession of the Gas- 

 teropoda and the Acephala shows, as we shall see in detail, an 

 analogous phenomenon. 



Some remarkable exceptions have been found to this rule 

 otherwise so general. These exceptions are precisely those 



