Darwin and Modern Science [139]
over the lower vertebrates, for lack of space to give them any adequate consideration, we may briefly take up the record of invertebrate life. From the overwhelming mass of material it is difficult to make a representative selection and even more difficult to state the facts intelligibly without the use of unduly technical language and without the aid of illustrations.
Several groups of the Mollusca, or shell-fish, yield very full and convincing evidence of their descent from earlier and simpler forms, and of these none is of greater interest than the Ammonites, an extinct order of the cephalopoda. The nearest living ally of the ammonites is the pearly nautilus, the other existing cephalopods, such as the squids, cuttle-fish, octopus, etc., are much more distantly related. Like the nautilus, the ammonites all possess a coiled and chambered shell, but their especial characteristic is the complexity of the "sutures." By sutures is meant the edges of the transverse partitions, or septa, where these join the shell- wall, and their complexity in the fully developed genera is extraordinary, forming patterns like the most elaborate oak-leaf embroidery, while in the nautiloids the sutures form simple curves. In the rocks of the Mesozoic era, wherever conditions of preservation are favourable, these beautiful shells are stored in countless multitudes, of an incredible variety of form, size and ornamentation, as is shown by the fact that nearly 5000 species have already been described. The ammonites are particularly well adapted for phylogenetic studies, because, by removing the successive whorls of the coiled shell, the individual development may be followed back in inverse order, to the microscopic "protoconch," or embryonic shell, which lies concealed in the middle of the coil. Thus the valuable aid of embryology is obtained in determining relationships.
The descent of the ammonites, taken as a group, is simple and clear; they arose as a branch of the nautiloids in the lower Devonian, the shells known as goniatites having zigzag, angulated sutures. Late in the succeeding Carboniferous period appear shells with a truly ammonoid complexity of sutures, and in the Permian their number and variety cause them to form a striking element of the marine faunas. It is in the Mesozoic era, however, that these shells attain their full development; increasing enormously in the Triassic, they culminate in the Jurassic in the number of families, genera and species, in the complexity of the sutures, and in the variety of shell-ornamentation. A slow decline begins in the Cretaceous, ending in the complete extinction of the whole group at the end of that period. As a final phase in the history of the ammonites, there appear many so-called "abnormal" genera, in which the shell is irregularly coiled, or more or less uncoiled, in some forms becoming actually straight. It is interesting to observe that some of these genera are not natural groups, but are "polyphyletic," i.e. are each derived from several distinct ancestral genera, which have undergone a similar kind of degeneration.
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
Several groups of the Mollusca, or shell-fish, yield very full and convincing evidence of their descent from earlier and simpler forms, and of these none is of greater interest than the Ammonites, an extinct order of the cephalopoda. The nearest living ally of the ammonites is the pearly nautilus, the other existing cephalopods, such as the squids, cuttle-fish, octopus, etc., are much more distantly related. Like the nautilus, the ammonites all possess a coiled and chambered shell, but their especial characteristic is the complexity of the "sutures." By sutures is meant the edges of the transverse partitions, or septa, where these join the shell- wall, and their complexity in the fully developed genera is extraordinary, forming patterns like the most elaborate oak-leaf embroidery, while in the nautiloids the sutures form simple curves. In the rocks of the Mesozoic era, wherever conditions of preservation are favourable, these beautiful shells are stored in countless multitudes, of an incredible variety of form, size and ornamentation, as is shown by the fact that nearly 5000 species have already been described. The ammonites are particularly well adapted for phylogenetic studies, because, by removing the successive whorls of the coiled shell, the individual development may be followed back in inverse order, to the microscopic "protoconch," or embryonic shell, which lies concealed in the middle of the coil. Thus the valuable aid of embryology is obtained in determining relationships.
The descent of the ammonites, taken as a group, is simple and clear; they arose as a branch of the nautiloids in the lower Devonian, the shells known as goniatites having zigzag, angulated sutures. Late in the succeeding Carboniferous period appear shells with a truly ammonoid complexity of sutures, and in the Permian their number and variety cause them to form a striking element of the marine faunas. It is in the Mesozoic era, however, that these shells attain their full development; increasing enormously in the Triassic, they culminate in the Jurassic in the number of families, genera and species, in the complexity of the sutures, and in the variety of shell-ornamentation. A slow decline begins in the Cretaceous, ending in the complete extinction of the whole group at the end of that period. As a final phase in the history of the ammonites, there appear many so-called "abnormal" genera, in which the shell is irregularly coiled, or more or less uncoiled, in some forms becoming actually straight. It is interesting to observe that some of these genera are not natural groups, but are "polyphyletic," i.e. are each derived from several distinct ancestral genera, which have undergone a similar kind of degeneration.
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