Page:Popular Science Monthly Volume 82.djvu/396

392 structure produced. What multitudes of animals are made out of essentially the same kinds of tissues! How limited in number those kinds are! The plants show the same lack of cellular diversity. Evolution has proceeded by means of new arrangements rather than new materials. This cellular stability, well fitting the needs of organisms, must have been fostered by selection. The nerve cell, the striated muscle cell, are astonishingly modified from ameboid ancestors, but the power that could do so much has left us only a few masterpieces. Is this the result of orthogenesis? Did development proceed along these lines, looking neither to the right nor to the left; or did selection oppose impassable barriers? Perhaps both, since orthogenetic trends may themselves be favored by selection.

Passing from tissues to organs and characters, we seem to find much greater, almost infinite, diversity. Recent research has, however, indicated the presence of determiners in the germ-plasm, factors which may be combined in endless ways in inheritance, but are themselves remarkably stable. It seems nearly certain that, so far from constantly presenting heritable variations, they rarely do so. This conclusion is based not merely on the Mendelian phenomena observed by experimenters, but also on the paleontological evidence. There are many groups of species, such as the oysters and the oaks, which have existed since Mesozoic times, producing innumerable species, but so far as we can see, practically all by the shuffling of characters present within the genus all along. Among the Unionidæ, the fresh-water mussels, Ortmann has recently commented on the occurrence of practically identical shell characters in different genera; while land snails afford a number of similar instances. In insects, these phenomena are constantly observed; types of color and marking are nearly the same in Lepidoptera of diverse structure; and in some of the Hymenoptera peculiar characters, such as spines on the cheeks, appear here and there as if at random. In one genus of bees the sexes differ in the character of the tongue, one having that organ pointed, the other having it obtuse; differences hitherto considered to mark families.

We are, therefore, led to see a certain stability amidst all the instability of the multicellular animals; a stability of types of tissue on the one hand, a stability of determiners on the other. Change in the stuff of which living things are made is not a common phenomenon; indeed, we know little or nothing about it. The experiments of Tower and MacDougal, in which heritable variations were apparently produced, can be explained rather on the supposition that certain determiners were destroyed than on the idea that they were altered.

Natural selection, it has often been said, creates nothing. It merely makes use of the variations already present. In Darwin's time, it was not appreciated that so many of the observable variations are due to the