THE POPULAR SCIENCE MONTHLY.
affinity passes in its lowest degree into the attraction of aggregation."
As to Mr. Launcelot's paragraph, wherein he states that many metals, other than gold, have been reduced to translucent tenuity, your note makes any answer from me unnecessary.George Iles.
THE PROGRESS OF AMERICAN BIOLOGY.
WE publish this month the first half of the able and interesting address delivered by Prof. Marsh, before the American Association, at Nashville, last August, on the "Introduction and Succession of Vertebrate Life in America," and which is the first complete edition that has appeared in any periodical. The paper is, from its nature, somewhat technical, but the author could not help that, as, in dealing with newly-discovered forms of life, he is compelled to use new terms a little freely, and to make his account complete he includes some lists of genera, which will be of great service to biological students, and should not frighten off unscientific readers, who will find much to interest them in the general treatment of the subject. There is nothing more remarkable in our time than the activity of its scientific thought, and the importance of the new results that are being reached in all the spheres of investigation. Of this, we have a striking illustration in the fact that four months was sufficient almost to antiquate Prof. Marsh's address, and make it necessary to post it up to the beginning of the present year, by notes from the author, stating what new things have been discovered since its delivery. Among these are mentioned a new species of fossil fish (Ceratodus), that has recently attracted much scientific attention, and is significant as the first found in the Mesozoic formations of this country. A number of new Jurassic reptiles, some of enormous size, and a new genus (Epihippus), a missing Eocene link in the genealogy of the horse, have also come to light. The interesting point here is, that these are all forms that the evolutionist was expecting from our marvelous Rocky Mountain region, and they show how rapidly the biological evidence of this theory is accumulating. The whole address is indeed a weighty contribution to the literature of this doctrine, as, besides the mere record of ancient life which it affords, the genealogies of many groups of animals are now traced for the first time. It is well known that Prof. Marsh, by his skill, enterprise, and assiduity, has made the field of the exploration of the Western fossil-beds very much his own; and in this address, which first attempts a summary statement of what is known of the extinct vertebrate life of this continent, he necessarily includes his own results. Among its leading features there is a discussion of the migrations of extinct mammals, and strong evidence is presented (in opposition to previous opinion upon the subject) that North America is really the oldest continent, from which South America, as well as Asia and Europe, derive many of their animals. The author's observations have led him also to conclude that there is at present no evidence that any of the supposed bird-tracks of the Connecticut River sandstone were made by birds, but were probably all made by reptiles. In discussing the unsettled question among geologists as to the line between the Cretaceous and Tertiary rocks in the West, Prof. Marsh puts forward a new principle or law as to the value of different kinds of fossils in determining geological age. His investigations show that the higher the grade of life, the better is the evidence