In the huge assembly of ammonites it is not yet possible to arrange all the forms in a truly natural classification, which shall express the various interrelations of the genera, yet several beautiful series have already been determined. In these series the individual development of the later general shows transitory stages which are permanent in antecedent genera.
To give a mere catalogue of names without figures would not make these series more intelligible.
The Brachiopoda, or "lamp-shells," are a phylum of which comparatively few survive to the present day; their shells have a superficial likeness to those of the bivalved Mollusca, but are not homologous with the latter, and the phylum is really very distinct from the molluscs. While greatly reduced now, these animals were incredibly abundant throughout the Palaeozoic era, great masses of limestone being often composed almost exclusively of their shells, and their variety is in keeping with their individual abundance. As in the case of the ammonites, the problem is to arrange this great multitude of forms in an orderly array that shall express the ramifications of the group according to a genetic system. For many brachiopods, both recent and fossil, the individual development, or ontogeny, has been worked out and has proved to be of great assistance in the problems of classification and phylogeny. Already very encouraging progress has been made in the solution of these problems. All brachiopods form first a tiny, embryonic shell, called the protegulum, which is believed to represent the ancestral form of the whole group, and in the more advanced genera the developmental stages clearly indicate the ancestral genera of the series, the succession of adult forms in time corresponding to the order of the ontogenetic stages. The transformation of the delicate calcareous supports of the arms, often exquisitely preserved, are extremely interesting. Many of the Palaeozoic genera had these supports coiled like a pair of spiral springs, and it has been shown that these genera were derived from types in which the supports were simply shelly loops.
The long extinct class of crustacea known as the Trilobites are likewise very favourable subjects for phylogenetic studies. So far as the known record can inform us, the trilobites are exclusively Palaeozoic in distribution, but their course must have begun long before that era, as is shown by the number of distinct types among the genera of the lower Cambrian. The group reached the acme of abundance and relative importance in the Cambrian and Ordovician; then followed a long, slow decline, ending in complete and final disappearance before the end of the Permian. The newly-hatched and tiny trilobite larva, known as the protaspis, is very near to the primitive larval form of all the crustacea. By the aid of the correlated ontogenetic stages and the succession of the adult forms in the rocks, many phylogenetic series have been established and a basis for the natural arrangement of the whole class has been laid.
Very instructive series may also be observed among the Echinoderms and, what is very rare, we are able in this sub-kingdom to demonstrate the derivation of one class from another. Indeed, there is much reason to believe that the extinct class Cystidea of the Cambrian is the ancestral group, from which all the other Echinoderms, star-fishes, brittle-stars, sea-urchins, feather-stars, etc., are descended.
The foregoing sketch of the palaeontological record is, of necessity, extremely meagre, and does not represent even an outline of the evidence, but merely a few illustrative examples, selected almost at random from an immense body of material. However, it will perhaps suffice to show that the geological record is not so hopelessly incomplete as Darwin believed it to be. Since "The Origin of Species" was written, our knowledge of that record has been enormously extended and we now possess, no complete volumes, it is true, but some remarkably full and illuminating chapters.
The main significance of the whole lies in the fact, that JUST INPROPORTION TO THE COMPLETENESS OF THE RECORD IS THE UNEQUIVOCAL CHARACTEROF ITS TESTIMONY TO THE TRUTH OF THE EVOLUTIONARY THEORY.
The test of a true, as distinguished from a false, theory is the manner in which newly discovered and unanticipated facts arrange themselves under it.
No more striking illustration of this can be found than in the contrasted fates of Cuvier's theory and of that of Darwin. Even before Cuvier's death his views had been undermined and the progress of discovery soon laid them in irreparable ruin, while the activity of half-a-century in many different lines of inquiry has established the theory of evolution upon a foundation of ever growing solidity. It is Darwin's imperishable glory that he prescribed the lines along which all the biological sciences were to advance to conquests not dreamed of when he wrote.