Description and keys to saw-fly and Lepidopterous larvæ. TOWNSEND, C. H. T. 1893. A General Summary of the Known Larval Food-habits of the Acalyptrate Muscidæ. Can. Ent., Vol. XXV, pp. 10-16. WARD, H. B., WHIPPLE, G. C., and others. Fresh Water Biology. (In press.) New York. This work consists of chapters by numerous specialists on the various groups of fresh-water animals including insects, gives keys for the determination of specimens, short chapters on their general biological relations, and references to the literature. VII. THE LAWS OF ENVIRONMENTAL CHANGE, OR THE “ORDERLY SEQUENCE OF EXTERNAL NATURE” THE DYNAMIC AND PROCESS RELATIONS OF THE ENVIRONMENT “Of all the truths relating to phenomena, the most valuable to us are those which relate to the order of succession. On a knowledge of these is founded every reasonable anticipation of future facts, and whatever power we possess of influencing those facts to our advantage.”--JOHN STUART MILL. “To study life we must consider three things: first, the orderly sequence of external nature; second, the living organism and the changes which take place in it; and, third, the continuous adjustment between the two sets of phenomena which constitutes life.”--W. K. BROOKS. “The truth, indeed, is, that in physical inquiries, the work of theory and observation must go hand in hand, and ought to be carried on at the same time, more especially if the matter is very complicated, for there the clue of theory is necessary to direct the observer. Though a man may begin to observe without any hypothesis, he cannot continue long without seeing some general conclusion arise; and to this nascent theory it is his business to attend, because, by seeking either to verify or to disprove it, he is led to new experiments, or new observations. He is led also to the very experiments and observations that are of the greatest importance, namely to those _instantiæ crucis_, which are the criteria that naturally present themselves for the trial of every hypothesis. He is conducted to the places where the transitions of nature are most perceptible, and where the absence of former, or the presence of new circumstances, excludes the action of imaginary causes. By this correction of his first opinion, a new approximation is made to the truth; and by the repetition of the same process, certainty is finally obtained. Thus theory and observation mutually assist one another; and the spirit of system, against which there are so many and such just complaints, appears, nevertheless, as the animating principle of inductive investigation. The business of sound philosophy is not to extinguish this spirit, but to restrain and direct its efforts.” --J. PLAYFAIR, “Illustrations of the Huttonian Theory of the Earth,” Edinburgh, 1802, pp. 524-525.