[Illustration: FIG. 232.--Butterfly and young.] [Illustration: FIG. 233.--Tortoise-shell butterfly.] When the butterfly is at rest its wings are held aloft, and many are so colored that in this position the wing resembles a leaf and the animal escapes observation. A marvelous example of this protective mimicry is observed in the East Indian butterfly, Kallima (Fig. 235). The wings have a little projection which resembles a stem from which a dark mark resembling a midrib extends. When the butterfly alights, this seeming stem, as shown in the illustration, appears to join to the branch, and the resemblance to a leaf is so perfect that the most careful observer is often deceived. Other butterflies observed by Wallace mimicked dry oak leaves and dead leaves of various kinds. All the spots and colors of decay were imitated in their wings. Other Indian forms resemble fungus, and utterly disappear as they alight upon it. No more attractive butterfly is seen than the finely marked Vanessa, the peacock butterfly (Fig. 236), which has beautiful peacock marks upon its wings in vivid blue. [Illustration: FIG. 234.--White butterfly and young.] [Illustration: FIG. 235.--A butterfly which resembles a leaf.] In southern California, almost every spring, there is a migration of butterflies from the south northward along the Sierra Madre. I have watched them for hours, numbers being seen over a given spot every moment. By writing to postmasters and other persons in different sections, I found that the migrating band was two hundred miles long and from ten to twenty miles wide. Doubtless this was but a fraction of its actual extent, it being made up, in reality, of millions of yellow butterflies. Darwin saw such a migration in South America. Their pathway was several miles in width, they filled the air like a yellow cloud, and were several hours passing a given point. Vessels out at sea have met with similar flocks blown away from the shore. [Illustration: FIG. 236.--Peacock butterfly.] [Illustration: FIG. 237.--Silkworm moth.]