Each advancing age was richer in both flora and fauna than the ages that had preceded it, because the soil, climate, and atmosphere of the earth became more and more favorable to both animal and vegetable existence. Nothing in all this succession lived or died in vain. The plants freed the air of carbon and enriched it with oxygen;[41] the decay of animal and vegetable matter increased and enriched the soil. FOOTNOTES: [41] Nature Reader, No. 3, Lessons 6, 7. LESSON XIX. THE BIRDS OF OTHER DAYS. “Thy flitting form comes ghostly dim and pale, As driven by a beating storm at sea; Thy cry is weak and scared, as if thy mates had shared The doom of us: thy wail--what does it bring to me?” --DANA. [Illustration: OLD FASHIONS IN FEATHERS.] It was an often-repeated saying of the great Linnæus that “Nature never makes a leap.” By this he meant that the law of nature is a law of gradations, and that in the long up-growth of organisms, the steps are short between the various classes of living things. The work of creation is harmonious, and moves from one grade of organization to another, with the easy transition of carefully shaded colors, or the ascent of the scale from note to note. But this creative harmony began very long ago, and in the innumerable ages which have rolled by with their train of living things, many types have been lost. Enough remain to fill us with wonder and admiration, when we see how the organism of one creature has been changed and adapted in some other succeeding animal. We quoted in the last lesson the remark, “a bird is but a glorified reptile”; and, far apart in type as we might at first glance consider an eagle and a crocodile, or a lizard and a dove, it is not very difficult to trace, by the help of fossil forms, the similarity of organism and the steps in the process of change. Reptiles and birds belong to the class vertebrates; the backbone is then the first distinguishing feature. In the reptile, the spinal column with well-defined vertebræ passes from the head to the tail.[42] In the bird we find the vertebræ of the neck well marked, and more numerous than in the reptile; the shoulder-plates, breast-bone and hip-bones are largely developed, forming the box-like body of the bird, and embracing the backbone. In a similar fashion we often find certain insects in which one of the rings of which the jointed body is composed is developed far more than any of the others.