Form and Function Part 38

The most valuable contribution made by palaeontologists 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. Cla.s.sical 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, Deperet 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 _Palaeomastodon 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. america.n.u.s_ 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" (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. .h.i.therto 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 of evolution. The American palaeontologists 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 palaeontologists, L. Rutimeyer (1825-1895) and V. O. Kowalevsky (1842-83), who seem to have carried on the splendid tradition of Cuvier.

Speaking of Kowalevsky's cla.s.sical memoir, _Versuch einer naturlichen Cla.s.sification 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 gra.s.s. In this survey of compet.i.tion 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 palaeontologists, including Osborn himself.

Osborn's law of "adaptive radiation," which links on to Darwin's law of divergence,[553] const.i.tutes 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.

"The most primitive mammals were probably small insectivorous or omnivorous forms, therefore with simple, short-crowned teeth, of slow-moving, ambulatory, terrestrial, or arboreal habit, and with short feet provided with claws. In seeking food and avoiding enemies in different habitats the limbs and feet radiate in four diverse directions; they either become _fossorial_ or adapted to digging habits, _natatorial_ or adapted to _amphibious_ and finally to _aquatic_ habits, _cursorial_ or adapted to swift-moving, terrestrial progression, _arboreal_ or adapted to tree life. Tree life leads, as its final stage, into

LIMBS AND FEET.

Volant.

/ Fossorial. Arboreal.

/ Short-limbed, plantigrade, } Ambulatory pentadactyl, unguiculate } or Stem. } Terrestrial.

/ Natatorial. Cursorial Amphibious. Digitigrade.

/ Aquatic Unguligrade.

TEETH.

Omnivorous.

{ Gra.s.s.

{ Fish. | { Herb.

Carnivorous { Flesh. | Herbivorous { Shrub.

{ Carrion. | / { Fruit.

| / { Root.

| / | / Myrmecophagous.

| / / Dent.i.tion reduced.

| / / | / / | / / |/ / Stem: Insectivorous.

the parachute types of the flying squirrels and phalangers, or into the true flying types of the bats.... Similarly in the case of the teeth, insectivorous and omnivorous types appear to be more central and ancient than either the exclusively carnivorous or herbivorous types. Thus the extremes of carnivorous adaptation, as in the case of the cats, of omnivorous adaptation, as in the case of the bears, of herbivorous adaptation, as in the case of the horses, or myrmecophagous adaptation, as in the case of the anteaters, are all secondary" (_loc. cit._, pp.

23-4).

We have now reached the end of our historical survey of the problems of form. What the future course of morphology will be no one can say. But one may hazard the opinion that the present century will see a return to a simpler and more humble att.i.tude towards the great and unsolved problems of animal form. Dogmatic materialism and dogmatic theories of evolution have in the past tended to blind us to the complexity and mysteriousness of vital phenomena. We need to look at living things with new eyes and a truer sympathy. We shall then see them as active, living, pa.s.sionate beings like ourselves, and we shall seek in our morphology to interpret as far as may be their form in terms of their activity.

This is what Aristotle tried to do, and a succession of master-minds after him. We shall do well to get all the help from them we can.

[519] See E. B. Wilson's masterly book, _The Cell in Development and Inheritance_, New York and London, 1900.

[520] _Q.J.M.S._, xxvi. 1886.

[521] _Wood's Holl Biological Lectures_ for 1893.

[522] _Arch. f. Ent.-Mech._, i., pp. 380-90, 1895.

[523] _Beitrage zur Kritik der Darwinschen Lehre_, Leipzig, 1898.

[524] See E. B. Wilson, "The Embryological Criterion of h.o.m.ology," _Wood's Holl Biological Lectures_, Boston, pp. 101-24, 1895; Braem, _Biol. Centrblt._, xv., 1895; T. H. Morgan, _Arch. f. Ent.-Mech._, xviii.; J. W.

Jenkinson, _Mem. Manchester Lit. Phil. Soc._, 1906, and _Vertebrate Embryology_, Oxford, 1913; A. Sedgwick, article "Embryology" in _Ency. Brit._, p. 318, vol. xi., 11th Ed. (1910).

[525] For a detailed treatment of this important point see the remarkable volume of E. Schulz (Petrograd), _Prinzipien der rationellen vergleichenden Embryologie_, Leipzig, 1910.

[526] "La Poecilogonie," _Bull. Sci. France et Belgique_, x.x.xix., pp. 153-87, 1905.

[527] _Un probleme de l'evolution. La loi biogenetique fondamentale_, Paris and Montpellier, 1908.

[528] _Vergleichung des Entwickelungsgrades der Organe zu verschiedenen Entwickelungszeiten bei Wirbeltieren_, Jena, 1891.

[529] Quoted by Keibel, _Ergebn. Anat. Entwick._, vii., p.

741.

[530] "Studien zur Entwickelungsgeschichte des Schweines,"

Schwalbe's _Morphol. Arbeiten_, iii., 1893, and v., 1895.

_Normentafeln zur Entwickelungsgeschichte des Schweines_, Jena, 1897.

"Das biogenetische Grundgesetz und die Cenogenese,"

_Ergebn. Anat. Entw._, vii., pp. 722-92, 1897.

"U. d. Entwickelungsgrad der Organe," _Handb. vergl.

exper. Entwick. der Wirbelthiere_, iii., 3, pp. 131-48, 1906.

[531] "Beitrage zur Embryologie der Wiederkauer," _Arch.

Anat. Entw._, 1889.

[532] "Die individ. Variation d. Wirbeltierembryo,"

_Morph. Arbeit._, v., 1895.

[533] "U. Variabilitat u. Wachstum d. embryonalen Korpers," _Morph. Jahrb._, xxiv., 1896.

[534] "Gastrulation u. Keimblatterbildung der _Emys lutaria taurica_," _Morph. Arbeit._, i., 1891.

"Kainogenese," _Morph. Arbeit._, vii., pp. 1-156, 1897, and also separately. _Biomechanik, erschlossen aus dem Prinzipe der Organogenese_, Jena, 1898.