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foraminifera are microscopic animals, mere bits of jelly, living in the sea, which produce many forms of beautiful microscopic shells. It is of these shells that common chalk is mainly composed. Even more remarkable is the amount of variation found in one group of sponges, of which Haeckel (the highest living authority in regard to this very group) remarks, –
“Just in proportion as the systematizer takes the ideas of genus, species, and varieties in a wider or narrower sense, he distinguishes in the little group of chalk-sponges, either only a single genus with three species, or three gerera with twenty-one species, or thirty-nine genera with two hundred and eighty-nine species, or even a hundred and thirteen genera with five hundred and ninety-one species. But all these diverse forms are so intimately connected by numerous transitions and intermediate forms, that the common descent of all the chalk-sponges from a single ancestral form, the olynthus, can be proved With certainty.” 1
Few students of the life-history of the earth can have failed to dwell with extreme interest upon the story of the development of species from species told by the rocks at Steinheim in Würtemberg, Germany. Here was once a small lake; and in its waters grew countless little shell-fish, many of them water-snails like those of lakes and rivers at the present day. By the appropriation of the dissolved limestone in the waters of the lake, generation after generation of these snails built up their shells, only to let them fall to the bottom on the death of the little inhabitant. Year after year the sand deposited on the lake-bed carried down with it and buried myriads of the little shells, which have remained in a very perfect state of preservation to the present time. These have been most carefully studied by expert palæontologists from all parts of the world, and there is a general agreement among many of the most competent of these investigators as to the essential lesson of the deposits. The forms of the upper strata have been derived by descent with modification from those of the lower (and older) strata. It would seem, too, from the nature of the deposits, that the whole series must have been laid down pretty rapidly, and therefore that the remarkable variations presented by the snails must have taken place within a very short period, geologically speaking. And not only are the shells of the lowest layer in the whole series so different from those of the uppermost one that (if the intermediate forms of shell had not been discovered) they
1 Haeckel, Descent of Man, i. p. 117. 2 Schmidt, Descent and Darwinisın, p. K.
would certainly be called different species; but there are many among the intermediate forms themselves, which, if they had been found separated from the others, would have been counted distinct species. The figures on page
to give some idea of the extraordinary amount of variation which these snails have undergone. The genealogy here suggested by Professor Hyatt is a hypothetical one, it is true, but founded upon an amount of investigation and a mass of evidence which must give his conclusions the greatest weight.
Each of the series I., II., III., IV., seems to have had its origin in the species which forms the lowest member of its column, i. e., Series I. from the form numbered 16, Series II. from No. 12, and so on. These original forms, 1, 8, 12, and 16 are all found in other deposits geologically older than those of Steinheim. It will be noticed at once that some of the series have developed much more freely than others. Series IV. seems to have met at Steinheim with extremely favorable conditions for its growth, and Series II. with unfavorable conditions. Each of the four family-trees here presented is shorn of all its minor twigs and branches, that is, very many intermediate forms actually occurring at Steinheim are, for simplicity's sake, omitted from the cut. The Steinheim beds themselves, with their interminable series of connecting links, binding together most widely divergent forms, furnish, indeed, an argument in favor of the hypothesis of speciesmaking by inherited variation which no series of illustrations, however full, can convey to the eye of the zoologist.
1 From a plate kindly loaned the authors by Professor Alpheus Hyatt.
Here, then, we have the record, preserved in that most trustworthy of all books, the book of nature, of the growth of a new species by gradual change from a former one ; unless, indeed, we prefer to suppose, that, for an indefinite number of times during the existence of the lake, the snails living at one period were killed off, and another lot created outright to take the place of the former generation. Evidently, to suppose this would be doing much as Voltaire did, when, in order to avoid the necessity for believing that the limestones of south-western Europe were originally formed beneath the waters of the sea, he claimed that the shells of which these lime
1 A very full and most interesting account of the Steinheim deposits is given by Professor Alpheus Hyatt, in his Genesis of Planorbis at Steinheim, Anniversary Memoirs of Boston Society Natural History, 1880.