chief interest of Dr. Pettigrew's book for thoughtful readers will consist in the skillful way he strikes through those diversities of movement and medium which are involved in the three forms of progression, and brings out the principles that are common to all. "We are apt," he says, "to consider walking as distinct from swimming, and walking and swimming as distinct from flying, than which there can be no greater mistake. Walking, swimming, and flying are, in reality, only modifications of each other. Walking merges into swimming, and swimming into flying, by insensible gradations. The modifications which result in walking, swimming, and flying, are necessitated by the fact that the earth affords a greater amount of support than the water, and the water than the air. That walking, swimming, and flying, represent integral parts of the same problem, is proved by the fact that most quadrupeds swim as well as walk, and some even fly, while many marine animals walk as well as swim, and birds and insects walk, swim, and fly, indiscriminately."
The problem thus becomes interesting from the unity of its fundamental laws, but for the author it has more than a speculative interest; it has a scientific importance as furnishing conditions for solving the problems of artificial progression.
Upon this point he remarks: "The history of artificial progression is essentially that of natural progression. The same laws regulate and determine both. The wheel of the locomotive and the screw of the steamship apparently greatly differ from the limb of the quadruped, the fin of the fish, and the wing of the bird; but, as I shall show in the sequel, the curves which go to form the wheel and the screw are found in the traveling surfaces of all animals, whether they be limbs (furnished with feet), or fins, or wings.
"It is a remarkable circumstance that the undulation or wave made by the wing of an insect, bat, or bird, when those animals are hovering before an object, and when they are flying, corresponds in a marked manner with the track described by the stationary and progressive waves in fluids, and likewise with the waves of sound.
"Of all animal movements, flight is indisputably the finest. It may be regarded as the poetry of motion. The fact that a creature as heavy, bulk for bulk, as are many solid substances can, by the unaided movements of its wings, urge itself through the air with a speed little short of that of a cannonball, fills the mind with wonder. Flight (if I may be allowed the expression) is a more unstable movement than that of walking and swimming, the instability increasing as the medium to be traversed becomes less dense. It, however, does not essentially differ from the other two, and I shall be able to show, in the following pages, that the materials and forces employed in flight are literally the same as those employed in walking and swimming."
These passages foreshadow the character of Dr. Pettigrew's book. He works out the principles of animal locomotion as a further step in the progress of artificial locomotion, by which the theoretical issues in the practical. After an elaborate analysis of the anatomical and dynamical conditions of flight, he goes into the question of its imitation by art, and points out the conditions on which he thinks the problem may be ultimately solved. Here, of course, he launches into an untried field, abounding with difficulty, and open to a diversity of opinions. Already a brisk controversy has sprung up in the London journals over his theory of flight, and the question of precedence in its elucidation between the French and the English; but, whatever may be its merits, the interest of Dr. Pettigrew's contributions to the question in the present volume will remain unaffected. We should not omit to state that the volume is profusely and beautifully illustrated with original cuts and plates.
Mr. Prang, having achieved fortune and fame in the cultivation of the chromo-lithographic art in the department of fancy pictures, has at length turned his attention to education, and applied it to the illustration of objects of natural history. A large num-