10 ELEMENTS OF PALAEONTOLOGY 



embryological investigations zoologists and botanists are able to trace out the 

 gradual development and differentiation of an organism through all its various 

 stages, and thereupon to construct a tree of descent (phylogcny) founded upon 

 the successive phases of growth, nevertheless such hypothetical genealogies 

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

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

 represented by corresponding fossil embryonic 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 palaeontology is from the nature of things 

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

 however, 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 to us, notwithstanding the imperfection of the 

 palaeontological record. By transitional 'series are meant a greater or lesser 

 number of similar forms occurring through several successive horizons, and 

 constituting a practically unbroken morphic chain. Oftentimes, indeed, the 

 differences between individuals belonging to different 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 palaeontological 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 Linnaean and Cuvierian' schools contended that each 

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

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

 those holding to the theory of descent, evolution, or transmutation, look 

 upon varieties, species, subgenera, genera, families, orders, classes, and sub- 

 kingdoms as distinctions of merely transient importance, 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 



