Page:Popular Science Monthly Volume 59.djvu/295

Rh It is obvious that animals with feeble powers of travel are of the greatest value in these researches, because they indicate more ancient and more enduring changes of sea and land than freely mobile creatures, such as birds and quadrupeeds, which may spread, conditions favoring, with great rapidity. Moreover, the invertebrates have changed much more slowly than higher animals; their evolution has been slow. Almost the whole great drama of mammalian evolution has been acted during Tertiary time, while there has scarcely been generic change in the mollusks! Mammals and birds reflect in their distribution the later earth movements, the invertebrates and fishes the earlier.

The Helix snails are particularly well adapted to show ancient faunal relationships, as they occur under one or another form in almost all lands. But it is only in the present decade that their anatomy has been understood, and the true relationships of the various groups of the family recognized. One of the most interesting developments of the anatomical study of land snails has been the demonstration of a close relationship existing between Helices of the Philippine Islands and eastern Asia and those of California, Mexico and the Greater Antilles.

Years ago Dr. Karl, Semper, in his Travels in the Philippine Archipelago, showed that the arboreal Helices of the Philippines are provided with a muscular sack containing a calcareous needle—the so called ’dart'—and surmounted by a mucous gland, the whole being

an appendage of the reproductive organs. In species of both China and Japan the same peculiarities are found in these organs. It was already known that the Helices of Europe have a similar sack and dart, but the associated mucous gland is split into finger-like tubes and removed from the sack to an adjacent duct. The function of the dart apparatus is not well understood. The snails thrust their darts into one another during the mating time, and hence the dart is believed to be an excitation organ.