10 ELEMENTS OF PALEONTOLOGY 



the successive phases of grovvth, nevertheless such hypothetical genealogies 

 can only be relied upon as truthful when they are substantiated by paleonto- 

 logical facts. And only in cases where the difFerent ontogenetic stages are 

 represented by corresponding fossil primitive or generalised types, which 

 appear in the same chronological order, and clothe the supposititious ancestral 

 tree with real forms, can the truthfulness of the latter be said to have been 

 established. This requirement paleontology is from the nature of things 

 unable to satisfy except in a few instances ; but a multitude of other facts 

 testifies to the blood-kinship between morphologically similar fossil and recent 

 organisms, and points to the direct descent of the younger from the older 

 forms. 



Geology proves conclusively that of the numerous floras and faunas which 

 lie buried in the rocks, those which are most nearly of the same geological 

 age bear the greatest resemblance to each other. It often happens that species 

 and genera occurring in a given formation reappear in the next following with 

 scarcely any perceptible changes, so that the doctrine of the gradual trans- 

 formation and transmutation of older forms is irresistibly forced upon one, 

 while the faunas and floras of later periods assert themselves as the obvious 

 descendants of the more ancient. Other weighty evidence for the progressive 

 evolution of organisms is afforded by fossil transitional series, of which a 

 considerable number are known, notwithstanding the imperfection of the 

 paleontological record. By transitional series are meant a greater or less 

 number of similar forms occurring through several successive horizons, and 

 constituting a practically unbroken morphic chain. Often the differences 

 between individuals belonging to diff'erent periods are so slight that we can 

 hardly assign to them the value of a variety. But let a number of such 

 mutations occur in succession, the end-members of the series become finally 

 so divergent as to constitute distinct species and genera. The most striking 

 and most numerous examples of transitional series naturally occur in types 

 peculiarly well fitted for preservation, such as mollusks, brachiopods, sea- 

 urchins, corals and vertebrates. Particularly remarkable among mollusks 

 are the closely linked transitional series in ammonites. Among vertebrates 

 transmutation proceeded far more rapidly than among invertebrates, and 

 accordingly, the successive members of a series are usually so divergent as to 

 require their assignment to separate genera. 



With increasing abundance of paleontological material, the more numerous 

 and more complete are the series of intermediate forms which are brought to 

 light. But the more extended our knowledge of transitional series the greater 

 is the difficulty we encounter in defining our conception of species. While the 

 older disciples of the Linnean and Cuvierian schools contended that each 

 separate species was created with a certain definite sum of fixed characters, 

 and remained incapable of any extensive modifications ; on the other band, 

 those holding to the Darwinian theory of evolution look upon varieties, 

 species, subgenera, genera, families, Orders, classes and phyla merely as 

 arbitrary yet useful and convenient distinctions, corresponding to the state of 

 our information at the present time ; it being assumed that by means of 

 gradual transmutation during the course of ages all organisms have become 

 evolved from a single primitive cell, or from a few primitive types. 



According to the Linne-Cuvier doctrine, a species is composed of individuals 

 which are directly descended from one another, or from common ancestors, 



