complete and intelligible explanation of all the facts of the case; the other not only has no direct evidence in its favour, but is totally opposed to all experience, and leaves the developmental features unexplained and inexplicable. I have selected the sole for special description because the facts of the case are well known, and the argument is a simple and easily-followed one. Other animals, however, would serve the purpose equally well, and would afford illustrations quite as striking of the aid given us by the Recapitulation theory in unravelling embryological problems. Thus, a crab and a lobster are animals closely agreeing with one another in essential structure, and clearly belonging to the same zoological group. The characteristic difference in form between the two is due to the fact that, while in the lobster the hinder part of the body, or "tail," is well developed, forming about half the length of the animal, and being used as a swimming organ; this "tail" is in the crab very greatly reduced in size, is of no use for swimming, and instead of projecting horizontally backwards, is carried bent forwards under the anterior part of the body, to which it is so closely fitted as to escape notice at first sight. Here again, as in the case of the flat fish, the structural differences may clearly be traced to difference in habit. Lobsters not merely walk about on the sea-bottom on their legs, but are able to swim freely in a backward direction, by powerful jerks of the tail. Crabs, on the other hand, walk but do not swim; and in them the tail, being no longer of use, has greatly diminished in size, and become rudimentary. Crabs, however, in their early stages of development, are free-swimming animals, and have tails quite as well developed and fully as large, relatively to the whole animal, as lobsters; and it is only after they have reached a certain size that they abandon their freeswimming habits, sink to the bottom, and henceforth move by walking only. The case is exactly parallel to that of the flat fish; and the Recapitulation theory explains the developmental history of a crab by saying that it is a repetition of the ancestral history of crabs in general: that crabs are descended from animals essentially similar to lobsters-i.e., from Macrurous ancestors, and that each crab passes through a lobster stage in its development, because of the inherited tendency that all animals have to climb up their own genealogical trees. (Fig. 13.) The evidence of the descent of crabs from Macrurous ancestors involves, moreover, the supposition that they came into existence later than the Macrura. This supposition is supported by the evidence of palæontology, for the Macrura are found as fossils in the Devonian and Carboniferous periods, and abundantly so in the Jurassic and Cretaceous, but are comparatively scanty in the Tertiary period; the Brachyura, or crabs, on the other hand, are very abundant in the Eocene and numerous in the Cretaceous, but doubtfully represented in the earlier periods. Good examples of recapitulation are found in Molluscs. The typical Gasteropod has a large spirally-coiled shell; the Limpet, however, has a conical shell, which in the adult animal shows no A FIG. 13. C Young and adult specimens of one of the swimming Crabs (Portunus), to illustrate the transition from the long-tailed to the short-tailed conditior. A.-The Zoæa stage, characterised by the great length of the spines on the cephalothorax; by the large size of the powerful rowing maxillipedes; and by the long, jointed tail. The larva is swimming in the direction of the long dorsal spine, the spines serving to guide its course. X 15 B.-The Megalopa stage. This is the typical Macrurous condition, comparable to that of an adult Lobster or Prawn. ×5 C.-The adult Crab. As compared with the Megalopa stage the cephalothorax has increased greatly in width, while the tail has become relatively smaller, and is carried turned forwards beneath the thorax.x sign of twisting, although the structure of the animal shows its affinity to forms with spiral shells. How ever, in its early stages of development the Limpet has a spiral shell, which is lost on the formation of the conical shell of the adult. Recapitulation is not confined to the higher groups of animals, and good examples are found among the Protozoa. One of the best instances is that of Orbitolites, one of the most complex of the Foraminifera, which, during its own growth and development, passes through the series of changes by which the discoidal type of shell is derived from the simpler spiral shell. This forms an instructive example, for, owing to the mode of growth by addition of new shelly matter, the older parts are retained often unaltered, and in favourable examples all stages can be determined by simple inspection of the adult shell. (Fig. 14.) The mode of growth of shells is important, since it gives an opportunity for comparing the paleontological and embryological records. In such a shell as that of the Nautilus the central chamber is the oldest and first formed one, to which the other chambers are added in succession. If then the development of the shell is a recapitulation of ancestral history, the central chamber should represent the palæontologically oldest form, and the remaining chambers in succession forms of more and more recent origin. In the shells of Ammonites it has been shown that such a correspondence between historic and embryonic development really exists. In the middle Jurassic deposits the older Ammonites are flattened and disc-like, with numerous ribs; in later forms the Shells of Peneroplis and of Orbitolites, members of the group of Porcellanous Foraminifera, illustrating the mode of transition from the spiral to the discoidal shell. A.-An adult Peneroplis shell. The shell is spiral and chambered, the later-formed chambers being very wide, and having a tendency to overlap the preceding ones. X 20 B.-A young Orbitolites shell, in the Peneroplis stage of development. The shell is spiral and chambered, the last-formed chambers having a more marked tendency to overlap the preceding ones than in Peneroplis. The last or marginal chamber in the specimen figured extends almost the whole way round. × 30. C.-An adult Orbitolites shell, seen edgeways, so as to show the thickness of the disc, and the marginal pores through which the pseudopodia are protruded during life. ×7 D.-An adult Orbitolites shell, seen full face. In the centre is the spiral nucleus, which is the oldest part of the shell, and was originally the only part present; then comes a part in which the successive chambers become wider and wider, and overlap the older part of the shell more and more completely, and finally the marginal and latest formed part, in which each chamber is circular and completely surrounds its predecessors. The fainter radial lines indicate the secondary partitions by which the chambers are subdivided. ×7 |