Page:Popular Science Monthly Volume 14.djvu/131

Rh possibility. With the habitual positions of our eyes, the axes are neither parallel nor too convergent, and we see dimly two images of our nose and objects too near our eyes. Now, when corresponding parts of two stereoscopic pictures are apart from one another only one inch and a half or so, and are distant from our eyes one foot or so, to combine the two pictures our eyes are only required to take one of their habitual positions; that is, the axes of the eyes need only to be so placed as if we were looking at an object distant from our eyes two feet or so: hence the effort required is very trifling, even when unaided by any instrument. But when the two pictures are so separated by a partition that the right eye can see only the right picture and the left eye the left picture, the combination of the two will take place as easily as with an ordinary stereoscope. This is, I think, chiefly owing to the fact that as long as the two pictures are seen separate, the picture seen by each eye is covered by a dim image of the partition seen by the other eye, and it is only when they combine that we have a clear and distinct view of the pictures.—

Natural Selection.—An interesting case of the operation of the law of natural selection and the survival of the fittest is recounted in the American Naturalist, by S. F. Clarke. Having obtained a number of the gelatinous egg-masses of one of our native salamanders, he placed them in large glass jars, where they developed rapidly. After their gills and balancers had developed, they emerged from the eggs and began their active life in the water. A difficulty now appeared—the author could not discover the proper kind of food. Upon watching the animals closely, however, he soon found that they were eating off one another's gills. Closer examination showed that among the many were a few individuals which, although they came from the same parents and were subjected to the same conditions while in the egg, were yet gifted with greater vigor than most of their fellows. These few stronger ones ate off the gills of many of the weaker, and at the same time were enabled to protect their own gills from mutilation. These favorable conditions, the large supply of food, and the better aeration of the blood, soon began to show their influence upon the growth of the favored individuals. Within a week or ten days from the time of emergence from the egg, these favored few were fifty per cent, larger than their weaker comrades who were born upon the same day. Their mouths had by this time so increased in size that, no longer satisfied with nibbling off the gills of their brethren, they now began to swallow them bodily. Soon they were ten or twelve times as great in length and bulk as their victims.

Carnivorous Caterpillars.—A striking peculiarity of the caterpillars of Patagonian Lepidoptera, namely, their cannibalism, is noticed by Prof. Carl Berg. All caterpillars in Patagonia, of whatever family or group, prey upon their own kind. He kept them in captivity, and found that, even with an abundance of food-plants at hand, they preferred to devour one another, "hair and hide;" they even tear open the cocoons and prey on the chrysalids. One was observed to devour in twenty-four hours six or seven individuals of its own species. This peculiarity of Patagonian caterpillars is thus explained by the author: During the summer there are extreme heat and drought in Patagonia, and these causes, together with the prevailing dry winds, parch the vegetation, scanty at best. The caterpillars are in consequence greatly straitened for food, and the struggle for life has led them to seek some other means of subsistence. Hence their cannibalism, which, being transmitted by heredity from generation to generation, becomes a second nature, and the practices to which they were at first driven by want they now perpetuate through habit.

A Battle-Royal among Ants.—F. E. Colenso, of Maritzburg, Natal, in a letter to Nature, gives an animated description of a fierce battle between ants, which he found engaged in mortal combat on his garden wall. Among the ants was a considerable number of larger individuals, "the soldier-ants" of the same species, and the whole ant community seemed to be bent on destroying them. A group of little ones would fasten on to a big one, the latter in the mean time making desperate efforts to