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rian species to the nautilus of the present time, yet it is not now possible to fill the many gaps which separate the several groups. It is possible, however, in the present state of paleontological knowledge to map out the life-bistory of the ammonites during a long portion of their most prosperous time, in such a way as to leave hardly any gaps in the record, and the following is a bare outline of the result.
At tlie beginning of the Jurassic period, the middle division of the Mesozoic era, ammonites of the family Arietidæ existed in the seas which covered the region which we now know as Central Europe. These ammonites were all of them mere varieties of one species, and that species (three of whose varieties are shown in Fig. 1 at the right, Fig. 1 at the middle, and Fig. 2 of the plate) was small and smooth. In the plate, the vertical columns represent lines of descent, extending thiough successive geological epochs, the oldest at the bottom. It will be noticed that from the three primitive forms, Figs. 1 and 2, rise a number of parallel series, and that from the sides of these latter spring occasional offshoots, like Fig. 17 and Fig. 28, which themselves in turn become, respectively, the radicals of new series of their own. Following the series through from bottom to top, it may at once be seen that, in general, each culminates in an especially large and ornate species, such, for example, as Fig. 8, Fig. 13, or Fig. 31, and then (undoubtedly from the influence of less favorable conditions of life) suffers degradation into the forms which, respectively, constitute the uppermost members of their series. These latest species show their imperfect nature sometimes by smaller size, as in Nos. 16 and 38, and generally by the loss of the square form of the aperture, the ribs, and the tubercles, as in the topmost members of every one of the series, which are apt to become smooth again.
1 Psiloceras planorbe.
The embryological stages of growth from the larval to the mature form have been studied in almost every variety of every species of ammonites figured in the plate, since the fossil remains of every such stage are found in abundance and usually quite perfect. The shell began to grow at the centre, which was at first very minute, and thus a history of the whole life of each shell is carried in its whorls. In an old shell, for example, one can study the characteristics of old age, which are very different from those of the aclult. The latter is found by breaking off
1 As seen in the views which represent the shell turned edgewise toward the eye.
the oldest (or outermost) whorls; and then by breaking off the arlult whorls a younger stage may be exposed for study, and so on until the tiny shell which enveloped the embryo may be obtained. This last shell is so small that it must be studied with a magnifying lens to find out its characteristics. As a result of this kind of study, it appears that in each series the earlier members of each species pass through a brief succession of larval changes to reach the comparatively simple adult form. The members which occupy the middle portion of a series pass through a more complete set of larval and subsequent changes, before the life-history of the individual is completed. The latest members of any series usually are characterized by an abbreviated life-history terminating in a form of shell not so much resembling that of the more prosperous members of the same series in their adult condition as in the condition of individuals which have survived into extreme old age and suffered consequent degeneration.
“Extraordinary and unforeseen correlations such as these, between chronological distribution and a biological classification founded upon the life-history of the individual, cannot be accidental. We have already shown, in preceding chapters, that our classification of series is natural and capable of verification by means of the cycles which are found to be present in the history of the individual and of the group. The process of verification does not, however, end with this, since approximately exact agreements may be found between the paleozoological and geological records wherever both classes of facts exist and have been minutely studied.”2 Remembering that the species and genera figured in the plate form only the most condensed sort of summary of the vast number of observed forms, shading off into one another by almost infinite gradations and represented by an unnumbered host of well-preserved specimens, assembled in the great paleontological collections of the world, the reader may perhaps conclude that no other such series of connecting links between genus and genus, species and species, is any where to be found, as that presented by the Arietidæ. Only a paragraph hiere
1 In other words, to use the valuable term which Professor Hyatt has invented to meet such cases, these uppermost members are geratologous forms.
1 i.e., Through the successive geological epochs which correspond to the members of the vertical series, as shown in the plate.
Hyatt, Genesis of the Arietidæ,
and there, out of the vast body of paleontological evidence in regard to the theory of descent, has been cited in the present chapter. It would appear that the general drift of the evidence is greatly in favor of the theory in question, though there are not wanting cases in which the evidence seems contradictory. In these instances it is not unfrequently fair to assume that what appears to have been a sudden introduction of species was not really such, but seems so, only because the book of geological records was never complete, and much of what was nce l'ecorded has since been erased. “Links," it is often said, must (if the development theory be true) once have existed, and ought now to be producible. But in the first place it is certain that sometimes there never was any
that is, the new species, or even genus, was produced abruptly from some other one, as in the case of the black-shouldered peacock, or of the brineshrimps already mentioned. Even where there was once a graduated series of forms, must it not usually have been true that this series, transitory in its nature, would fail to become represented by fossils ?
No domestic animal is more abundant than the dog, or more widely distributed; yet what probability is there that fossil remains are even