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THE POPULAR SCIENCE MONTHLY.

pungent herbs are noxious to them; but nettles are a favorite haunt of certain small land-shells. Spring is the most active season for snails; they attain their fullest development toward midsummer; and as winter approaches they penetrate the ground or in warm regions attach themselves to the bark of trees or to stones for a period of hibernation. They close the aperture of the shell with a leathery secretion, sometimes strengthened by more or less limy matter, or, if naked, may surround themselves with it like a cocoon.

 

Value of Scientific Amateurs.—Is it not true (asked Prof. Arthur Schuster in the British Association) that the one distinctive feature which separates England from all other countries in the world is the prominent part played by the scientific amateur, and is it not also true that our modern system of education tends to destroy the amateur? By amateur I do not necessarily mean a man who has other occupations and only takes up science in his leisure hours, but rather one who has had no academical training, at any rate in that branch of knowledge which he finally selects for study. We may, perhaps, best define an amateur as one who learns his science as he wants it and when he wants it. I should call Faraday an amateur. He would have been impossible in another country; perhaps he would be impossible in the days of the Science and Art Department. Other names will occur to you, the most typical and eminent being that of Joule. We are in danger of losing one great and necessary factor in the origination of scientific ideas. If I am right, there is a distinct advantage in having one section of scientific men beginning their work untrammeled by preconceived notions, which a systematic training in science is bound to instill. If school examinations could be more general, if scientific theories could only be taught at an age when a man is able to form an independent judgment, there might be some hope of retaining that originality of ideas which has been a distinctive feature of this country, and enabled our amateurs to hold a prominent position in the history of science. At present a knowledge of scientific theories seems to me to kill all knowledge of scientific facts. It is by no means true that a complete knowledge of everything that has a bearing on a particular subject is always necessary to success in an original investigation. In many cases such knowledge is essential, in others it is a hindrance. Different types of men incline to different types of research, and it is well to preserve the dual struggle.

 

What caused the Ice Age?—In their paper in the British Association on The Cause of the Ice Age, Messrs. P. F. Kendall and J. W. Gray maintain that the Glacial period came on with extreme slowness; that it was of long duration; that it passed away very abruptly and very recently, probably about ten thousand years ago; and that the geological record, though yielding evidence of ancient glaciers, yet was without trace of any previous Glacial period. They criticised the existing theories of the cause of the Ice age, and urged that the ingenious theory of Croll was objectionable upon several grounds. It was linked with a chronology which, even with the reservations made by Sir Robert Ball, was not reconcilable with geological facts. It involved the occurrence of repeated Glacial periods, and accounted neither for the very gradual approach nor the very abrupt departure of the cold. The theory of Mr. Upham, of the United States Geological Survey, that a great series of continental uplifts had raised enormous areas of the Northern Hemisphere above the snow-line, was based on evidence valid in itself, but failed to prove that the uplift was synchronous or coincided with the Glacial period. Further, there was irrefragable evidence that the British Isles stood at almost absolutely the same level as at present. Enormous ice-sheets swathed the whole of the northern and western portions of Britain. An explanation which would not apply to the British Isles might safely be rejected. The authors, although they formulate no theory of their own, invite the attention of astronomers to the suggestion that as the sun has undergone a secular cooling such as the president, Sir Archibald Geikie, declared in his address had happened to the earth, the Glacial period was a consequence of this cooling. This would perhaps account for the gradual refrigeration, leaving not the Ice age, but the genial period which suddenly supervened, to be accounted for. Varia-