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 (phylogeny) 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 
