Page:Popular Science Monthly Volume 78.djvu/146

142 Italy and central Europe. Hyatt carefully traces out the complicated genealogy of the related forms and describes their migrations, and his monograph is beautifully illustrated by heliographic plates. In order to carry out these studies, he visited the museums of Stuttgardt, Tübingen, Würzburg, Munich, Zurich, Paris, Semur, the British and Geological Society Museums of London, and other places in Europe.

In 1872-73, Hyatt lived in Würtemberg in order to study ammonites and also variations and evolution of the fossil snail shells Planorbis from the ancient Tertiary lake at Steinheim. As is well known, this lake gradually filled with gravel and limestone mud, and thus the later shells lived at higher and higher levels until the lake became wholly dry. Hyatt agrees with Hilgendorf that all of the species of Planorbis found at Steinheim are descended from four varieties of Planorbis levis which entered the lake in early times. At first hybrids were developed between these four varieties, but as the original stocks diverged more and more one from another, these hybrids died out. Hyatt finds that in all four stocks there is at first a tendency to increase the spiral of the shell due to a deepening of the lower at the expense of the upper umbilicus, thus eventually producing more or less trochiform shells. Hyatt states that the Steinheim shells develop similar species in many separate and distinct genetic series, and these parallelisms he ascribes to the fact that all lived in one and the same environment and were subjected to similar external influences. His genealogical series differ considerably from those of Hilgendorf, and the Steinheim shells must be restudied by some unbiased investigator before we can be certain of the facts in the controversy. Hyatt found that the young shells are always smooth, but in one race transverse ridges appear on the outer whorl and finally affect the inner whorls of their descendants. Uncoiling also appears first in the outer whorl and finally the inner whorls also uncoil, and in another stock a keel-like ridge forms first on the outer and afterwards extends to the inner whorls. Thus these characters are accelerated, i. e., appear earlier and earlier in the lives of the descendants. Hyatt concludes that the modifications of the Steinheim shells are due to the law of heredity with acceleration, and are not controlled by natural selection, although natural selection may have caused the dying out of the hybrids between the four original varieties. He also believes that gravity produces modifications of structure, and that unfavorable conditions cause uncoiling, produce transverse ridges and diminish the size of the shells. He states also that "the tendency to earlier and earlier inheritance in successive generations is apparently the result of disturbing and modifying agencies acting from without."

For the last twenty years of his life Hyatt studied the mutations and migrations of those most interesting of variable snails the Achatinelidæ of the Hawaiian Islands, and, indeed, he was upon the point of