V. Spider on Flower. (_Drawn by Miss L. Sullivan from a plate in McCook’s “American Spiders.”_) 144 VI. Elaps and Erythrolamprus. (_Modified by Mr. J. L. Ridgway from a figure in Romanes’ “Darwin and After Darwin.”_) 146 VII. Salamandra maculosa. (_Drawn by Mr. J. L. Ridgway from specimens in the United States National Museum._) 148 VIII. Paradise Birds. (_Drawn by Mr. J. L. Ridgway from specimens in the Smithsonian Institution._) 152 IX. Primrose Flowers. (_Drawn by Miss L. Sullivan from specimens._) 162 X. Bee Fertilizing Flower. (_Drawn by Miss L. Sullivan from specimens in the United States Bureau of Entomology._) 164 PLATES ENGRAVED IN BLACK AND WHITE IV. Leaf-hoppers. (_Drawn by Miss L. Sullivan from drawings collected by Dr. L. O. Howard._) 140 XI. Babies’ Grasping Power. (_From a Photograph taken by Dr. Louis Robinson._) 192 XII. Brain of Man. (_From Carus’ “The Soul of Man.”_) 202 INTRODUCTION. It is extremely difficult to realize the variety, wealth, and grandeur of animal and plant life as they exist on the globe to-day. Even if we endeavor to recall, in imagination, all that we have seen in streams and woodland, in ponds and rivers, in meadows, and in the air; even when we call to mind the multifarious specimens we have beheld in all the museums of natural history we have visited, and remember the most vivid descriptions that we have ever read of the wealth of tropical life;—even then we have only the faintest conception of the multitude and variety of _living forms_, not to mention the _vanished hosts of bygone ages_. It has always been the endeavor of students of nature to reduce this great host of living creatures to order by some system of classification. First one system of classification was adopted, and then another, with the progress of Botany and Zoölogy, until eventually, before the theory of evolution was entertained by scientific men, that system was adopted which naturalists likened to a tree. In this system, those lowest organisms which cannot properly be called either animals or plants, may be represented by a short trunk. Soon this short trunk divides into two large trunks, one of which represents the animal kingdom and the other the vegetable kingdom. Each of these trunks then sends off large branches representing classes, from which smaller and more numerous branches, representing orders, are given off. From these, other branches and sub-branches are separated, representing families and genera, and finally the terminal twigs or leaves represent species.