WHEN I said in NATURE (April 22) that Herbert Spencer was the first to apply the principle of similitude to dynamical problems in biology, I spoke in haste. I might have remembered that Borelli had shown, by help of this principle, that a man would never be able to fly by his own muscular power, and why (for instance) small animals are more active and leap higher than big ones. But I was quite ignorant of the fact that Galileo had treated the whole subject on the broadest lines and with the utmost clearness. His discussion will be found in the âœDialogues concerning Two New Sciences, admirably translated by Prof. Henry Crew and Alfonso de Salvio (New York: The Macmillan Co.; London: Macmillan and Co., Ltd., 1914). So numerous and interesting are the subjects dealt with in this wonderful book that the writer of a long and laudatory notice in NATURE (December 24, 1914) had not time or space to mention that the principle of similitude and the subject of animal mechanics are alluded to therein. The following extract (op. cit., p. 130) is but a small part of what Galileo has to say upon the principle of similitude:—