The contributions of palæontology to the solution of the problems of descent posed by morphology are, however, not all of this negative character. The law of recapitulation is in some well-controlled cases triumphantly vindicated by palæontology. Thus Hyatt and others found that in Ammonites the first formed coils of the shell often reproduce the characters belonging to types known to be ancestral, and what is more they have demonstrated the actual occurrence of the phenomenon known as acceleration or tachygenesis, often postulated by speculative morphologists.[546] This is the tendency universally shown by embryos to reproduce the characters of their ancestors at earlier and earlier stages in their development.
The most valuable contribution made by palæontologists to morphology and to the theory of evolution arose out of the careful and methodical study of the actual succession of fossil forms as exemplified in limited but richly represented groups. Classical examples were the researches of Hilgendorf[547] on the evolution of Planorbis multiformis in the lacustrine deposits of Steinheim, those of Waagen[548] on the phylogeny of Ammonites subradiatus, and the work of Neumayr and Paul[549] on Paludina (Vivipara).
These investigations demonstrated that it was possible to follow out step by step in superjacent strata the actual evolution of fossil species and to establish the actual "phyletic series."
To take an example from among the Vertebrates, Depéret has shown (loc. cit., pp. 184-9), that the European Proboscidea, belonging to the three different types of the Elephants, Mastodons and Dinotheria, have evolved since the Oligocene epoch along five distinct but continuous lines. The Dinotherian stock is represented at the beginning of the Miocene by the relatively small form D. cuvieri; this changes progressively throughout Miocene times into D. laevius, D. giganteum, and D. gigantissimum. Among the Mastodons two quite distinct phyletic series can be distinguished, the first commencing with Palæomastodon beadnelli of the Oligocene, and evolving between the Miocene and Pliocene into Mastodon arvernensis, after traversing the forms M. angustidens and M. longirostris, the second starting with the M. turicensis of the Lower Miocene and evolving through M. borsoni into the M. americanus of the Quaternary. The phyletic 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" (Depéret, 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 palæontology 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" (Depéret, p. 249).
It is one of the paradoxes of biological history that the palæontologists 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 of evolution. The American palæontologists in particular—Cope, Hyatt, Ryder, Dall, Packard, Osborn—have worked out a complete neo-Lamarckian theory based upon the fossil record.
The functional point of view was well to the fore in the works of those great palæontologists, L. Rütimeyer (1825-1895) and V. O. Kowalevsky (1842-83), who seem to have carried on the splendid tradition of Cuvier. Speaking of Kowalevsky's classical memoir, Versuch einer natürlichen Classification der fossilen Hufthiere, Osborn[552] writes:—"This work is a model union of the detailed study of form and function with theory and the working hypothesis. It regards the fossil not as a petrified skeleton, but as having belonged to a moving and feeding animal; every joint and facet has a meaning, each cusp a certain significance. Rising to the philosophy of the matter, it brings the mechanical perfection and adaptiveness of different types into relation with environment, with changes of herbage, with the introduction of grass. In this survey of competition it speculates upon the causes of the rise, spread, and extinction of each animal group. In other words, the fossil quadrupeds are treated biologically—so far as is possible in the obscurity of the past" (p. 8). The same high praise might with justice be accorded to the work of Cope on the functional evolution of the various types of limb-skeleton in Vertebrates, and on the evolution of the teeth as well as to the work of other American palæontologists, including Osborn himself.
Osborn's law of "adaptive radiation," which links on to Darwin's law of divergence,[553] constitutes a brilliant vindication of the functional point of view. "According to this law each isolated region, if large and sufficiently varied in its topography, soil, climate, and vegetation, will give rise to a diversified mammalian fauna. From primitive central types branches will spring off in all directions, with teeth and prehensile organs modified to take advantage of every possible opportunity of securing food, and in adaptation of the body, limbs and feet to habitats of every kind, as shown in the diagram [on p. [363]]. The larger the region and the more diverse the conditions, the greater the variety of mammals which will result.