Page:EB1911 - Volume 20.djvu/642

Rh nuances, or grades of difference, which are the more gradual the more finely we dissect the geologic column, while the terms species, sub-species and variety are generally based upon a sum of changes in several characters. Thus palaeontology has brought to light an entirely new nomenclatural problem, which can only be solved by resolutely adopting an entirely different principle. This revolution may be accomplished by adding the term “mutation ascending” or “mutation descending” for the minute steps of transformation, and the term phylum, as employed in Germany, for the minor and major branches of genetic series. Bit by bit mutations are added to each other in different single characters until a sum or degree of mutations is reached which no zoologist would hesitate to place in a separate species or in a separate genus.



The minute gradations observed by Hyatt, Waagen and all invertebrate palaeontologists, in the hard parts (shells) of molluscs, &c., are analogous to the equally minute gradations observed by vertebrate palaeontologists in the hard parts of reptiles and mammals. The mutations of Waagen may possibly, in fact, prove to be identical with the “definite variations” or “rectigradations” observed by Osborn in the teeth of mammals. For example, in the grinders of Eocene horses (see Plate III., fig. 8; also fig. 9) in a lower horizon a cusp is adumbrated in shadowy form, in a slightly higher horizon it is visible, in a still higher horizon it is full-grown; and we honour this final stage by assigning to the animal which bears it a new specific name. When a number of such characters accumulate, we further honour them by assigning a new generic name. This is exactly the nomenclature system laid down by Owen, Cope, Marsh and others, although established without any understanding of the law of mutation. But besides the innumerable characters which are visible and measurable, there are probably thousands which we cannot measure or which have not been discovered, since every part of the organism enjoys its gradual and independent evolution. In the face of the continuous series of characters and types revealed by palaeontology, the Linnaean terminology,

which is essentially based on a theory of interrupted or discontinuous characters, is inapplicable.

Embryology and Ontogeny.—In following the discovery of the law of recapitulation among palaeontologists we have clearly stated the chief contribution of palaeontology to the science of ontogeny—namely, the correspondences and differences between the individual order of development and the ancestral order of evolution. The mutual relations of palaeontology and embryology and comparative anatomy as means of determining the ancestry of animals are most interesting. In tracing the phylogeny, or ancestral history of organs, palaeontology affords the only absolute criterion on the successive evolution of organs in time as well as of (progressive) evolution in form. From comparative anatomy alone it is possible to arrange a series of living forms which, although structurally a convincing array because placed in a graded series, may be, nevertheless, in an order inverse to that of the actual historical succession. The most marked case of such inversion in comparative anatomy is that of Carl Gegenbaur (1826-1903), who in arranging the fins of fishes in support of his theory that the fin of the Australian lung-fish (Ceratodus) was the most primitive (or Archipterygium), placed as the primordial type a fin which palaeontology has proved to be one of the latest types if not the last. It is equally true that palaeontological evidence has frequently failed where we most sorely needed it. The student must therefore resort to what may be called a tripod of evidence, derived from the available facts of embryology, comparative anatomy and palaeontology.

The modes of change among animals, and methods of analysing them.—As historian the palaeontologist always has before him as one of his most fascinating problems phylogeny, or the restoration of the great tree of animal descent. Were the geologic record complete he would be able to trace the ancestry of man and of all other animals back to their very beginnings