Page:Popular Science Monthly Volume 48.djvu/636

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We can hardly conceive a more fascinating subject than the one treated of in this book.[1] Birds, by their song, their marvelous flight, their mysterious migrations, the independent intelligences among them, which have led in past times to many species becoming the associates of man, offer a most delightful subject for study. It is interesting to recall the fact that Cuvier and his contemporaries regarded the birds as a closed type, a group that seemed completely isolated from the other classes of vertebrates, and now they are known to be closely related to the reptiles. Mr. Headley has devoted considerable space in his book to these reptilian affinities. In his chapter on the embryo bird he might have added other reptilian characters, such as the claws which are seen on the fingers on the embryo sea-pigeon; particularly he might have described more fully the tarsal bones, showing that the ascending process of the astragalus had an independent center of ossification, and represented in the embryo the intermedium wedged in between the distal end of the tibia and fibula as shown in the salamanders. Birds may be cited for almost every point defined by Darwin in his theory of natural selection, not only in their reptilian affinities, as shown in their embryological and paleontological history, but in the very remarkable examples of variation in habit?, color markings, albinism, molting, sexual selection, protective coloration, etc. The author's ideas of homology and analogy are somewhat obscure. Having correctly defined analogy by the common example of a bird's wing and an insect's wing as being alike in function, but different in origin of structure, and hence analogous and not homologous, he proceeds to say that there is no homology between the wing of a bird and the wing of a bat; whereas there is the strictest homology in origin, structure, and function. In the next sentence he says that the tails of all vertebrates are homologous, which is correct. Yet here we have a few vertebræ in one tail and over fifty in another. We have the broad fin of a fish, the trowel of the beaver, and the climbing appendage of the monkey, and all are indeed homologous. In the same way limbs of the vertebrates are homologous; whether it be the pectoral fin of a fish, the leg of a turtle, the wing of a pterodactyl, of a bird or a bat, or the arm of man, a strict homology runs through them all. Mr. Headley has managed his material admirably, and one gets in a condensed form many facts about birds which have been brought to light within recent years. The clearness of the illustrations gives an added value to the book.

A difficult problem can be solved only by attacking it from one side after another. Prof. Donaldson has made his contribution to ascertaining the nature and action of the mind by setting forth the mode of growth of the chief portion of the nervous system.[2] He introduces his specific subject by three chapters discussing the general phenomena of growth and the rate of increase of the whole body. Then taking up the brain and spinal cord, he presents statistics showing at what rates these organs increase in weight and what weights they attain. He finds that the facts now available "contribute mainly to a healthy skepticism concerning the current interpretations of brain-weight." The author next traces the growth of the nerve elements and describes the architecture of the nervous system, calling attention to the changes of structure that are due to the growth of the organism. The process of dissolution in old age is also traced. Chapters on localization of function, physiological rhythms, and fatigue furnish additional data for the author's discussion of the subject of education, which concludes the volume. From these data he finds that "education must fail to produce any fundamental changes in the nervous organization, but to some extent it can strengthen formed structures by exercise, and in part waken into activity the unorganized remnant of the dormant cells, . . . On neurological grounds, therefore,

  1. The Structure and Life of Birds. By F. W. Headley, M. A. Pp. 417, with Illustrations. New York: Macmillan & Co. Price, $2.
  2. The Growth of the Brain. By Henry Herbert Donaldson. The Contemporary Science Series. Pp. 374, 12mo. London: Walter Scott, Ltd. Price, 3s. 6d. New York: Charles Scribner's Sons. Price, $1.25.