Although geologists and botanists have defined hundreds of thousands of species, they have not as a matter of fact until very recently attempted to secure exact measurements of differences, and the committee of the Association on ‘The Quantitative Study of Variation,’ of which Prof. Chas. B. Davenport is the recorder, aims to encourage such work. It is now over forty years since the facts and arguments presented in Darwin’s ‘Origin of Species’ paved the way for general acceptance of the doctrine of evolution. But the objection is hardly less valid to-day than it was then that the evidence for evolution is almost wholly indirect. Over and over again naturalists have been challenged to cite one case where a species in nature has changed within historic times and repeatedly they have taken refuge in the plea that the historic period is too short for a noticeable change to have taken place. This plea can be accepted, however, only so long as we have no exact way of measuring race change. When we can express quantitatively the condition of a community to-day, we may hope to be able to say whether any change has occurred after five, ten, or a hundred years. The committee of the Association has especially concerned itself with a piece of work which may be considered typical. In the headwaters of the Tennessee River there lives a univalor mollusc which is found nowhere else in the world and which belongs to a family of molluscs that was early separated from its marine cogeners as a fluviatile species. This genus, Io, varies greatly in different parts of the Tennessee basin. In some places it is smooth; in others, spiny; in others, long drawn out. Under a grant of the Association, Mr. C. C. Adams, of Bloomington, Ill., visited this region; travelled down one of the tributaries in a boat, collecting samples from every community of Ios; and went by train up a second river collecting at every stopping place. The results of this trip were, in a word, that in passing from the mouth to the headwaters of the two parallel tributaries the shells vary in parallel fashion and show a uniform, continuous change from the spiny, elongated condition characteristic of the mouths of the rivers to the smooth, more globose condition characteristic of the headwaters. The additional grant by the Association of one hundred dollars will assist Mr. Adams in making further quantitative studies on variation in the genus Io. * * * * *