yletic series of the true
elephants in Europe are relatively short, and go back only to the
Quaternary, _Elephas antiquus_ giving origin to the Indian elephant, _E.
priscus_ to the African.

The careful study of phyletic series brought to light the significant
fact that these lines of filiation tend to run for long stretches of
time parallel to, and distinct from one another, without connecting
forms. This is clearly exemplified in the case of the Proboscidea, and
many other examples could be quoted. Almost all rich genera are
polyphyletic in the sense that their component species evolve along
separate and parallel lines of descent.[550] "Such great genera as the
genus _Hoplites_ among the Ammonites, the genus _Cerithium_ among the
Gastropoda, the genus _Pecten_ or the genus _Trigonia_ among the
Lamellibranchs, each comprise perhaps more than twenty independent
phyletic series" (Deperet, p. 200).

Variation along the phyletic lines is gradual[551] and determinate, and
appears to obey definite laws. The earliest members of a phyletic series
are usually small in size and undifferentiated in structure, while the
later members show a progressive increase in size and complexity. Rapid
extinction often supervenes soon after the line has reached the maximum
of its differentiation.

The general picture which palaeontology gives us of the evolution of the
animal kingdom is accordingly that of an immense number of phyletic
lines which evolve parallel to one another, and without coalescing,
throughout longer or shorter periods of geological times. "Each of these
lines culminates sooner or later in mutations of great size and highly
specialised characters, which become extinct and leave no descendants.
When one line disappears by extinction it hands the torch, so to speak,
to another line which has hitherto evolved more slowly, and this line in
its turn traverses the phases of maturity and old age which lead it
inevitably to its doom. The species and genera of the present day belong
to lines that have not reached the senile phase; but it may be surmised
that some of them, _e.g._ elephants, whales, and ostriches, are
approaching this final phase of their existence" (Deperet, p. 249).

It is one of the paradoxes of biological history that the
palaeontologists have always laid more stress upon the functional side of
living things than the morphologists, and have, as a consequence, shown
much more sympathy for the Lamarckian theory