When Agassiz arrived in this country, there were many good observers of Nature here, and many who had accumulated a large store of facts. Each one had been working in his own way, almost alone, scarcely knowing the ultimate aims of scientific research, much less knowing how to arrive at them. To him, more than to any other person, zooelogists in this country are indebted for showing them how to work, and for presenting to them a plan to be worked out, with processes and means by which this is to be done. And now he designs to diffuse these high aims and methods throughout the community. As he says, "The time has come when scientific truth must cease to be the property of the few, when it must be woven into the common life of the world." Of all men, he is the one to gain the ear and understanding of the public on such matters, and to command the recognition of his conclusions. His faculty of simplifying great principles, and of clothing them in such language and with such illustrations as to render them intelligible and attractive to the uninstructed, is one of Professor Agassiz's most rare characteristics. In these chapters he has unfolded some of the methods by which high scientific results have been and may be attained, and has well illustrated them. In a short sketch of the progress of Natural History, he has noticed the methods which were successively pursued in its study, and the long time which elapsed before anything like true science was developed; he has pointed out the necessity and nature of classification, the important terms employed, as classes, orders, families, genera, and species, and their signification, and dwells upon the great idea that all the denominations represented by these terms exist definitely in Nature, and can be legitimate and permanent only as they conform to the plan laid down by Nature herself. Much of the work is devoted to the enforcement of this doctrine. He shows us, more especially by the class of Radiates, how objects at first view widely different all conform to the same definite plan, and how some which during a part of their history would not be suspected of having any alliance with each other, yet, by alternate generations, come to be identical. He shows, by the ovarian egg, the great simplicity and apparent identity of the beginnings of all animal life, and the successive steps by which the diversified forms of animals are developed, and insists upon the necessity of following the history of an animal through all its phases before its true place in the grand plan can be determined. He discusses the permanence of species, and the limits of their variation, which he illustrates more especially by the growth of corals, and most emphatically expresses his dissent from the startling development-doctrines of Darwin. But it would be fruitless to attempt an abstract of the numerous truths he has alluded to, and the methods by which such truths are to be sought. It is to these truths, in