The present volume offers from the classic pages of Darwin his summary of the argument of "The Origin of Species," his account of how that book came to be written, and his recapitulation of "The Descent of Man." All this affords a supreme lesson as to the value of observation with a purpose. When Darwin was confronted with an organ or trait which puzzled him, he was wont to ask, What use can it have had? And always the answer was that every new peculiarity of plant, or beast, is seized upon and held whenever it confers advantage in the unceasing conflict for place and food. No hue of scale or plume, no curve of beak or note of song, but has served a purpose in the plot of life, or advanced the action in a drama where the penalty for failure is extinction. As Charles Darwin stood first among the naturalists of the nineteenth century, his advocacy of evolution soon wrought conviction among the thinkers competent to follow his evidence and weigh his arguments. The opposition to his theories though short was sharp, and here he found a lieutenant of unflinching courage, of the highest expository power, in Professor Huxley. This great teacher came to America in 1876, and discoursed on the ancestry of the horse, as disclosed in fossils then recently discovered in the Far West, maintaining that they afforded unimpeachable proof of organic evolution. His principal lecture is here given. In a remarkable field of "natural selection" Bates, Wallace and Poulton have explained the value of "mimicry" as an aid to beasts, birds, insects, as they elude their enemies or lie unsuspected on the watch for prey. The resemblances thus worked out through successive generations attest the astonishing plasticity of bodily forms, a plasticity which would be incredible were not its evidence under our eyes in every quarter of the globe. Insects have high economic importance as agents of destruction: we are learning how to pit one of them against another, so as to leave a clear field to the farmer and the fruit grower. In this department a leader is Professor Howard, who contributes a noteworthy chapter on the successful fight against the pest which threatened with ruin the orange groves of California.