Page:Popular Science Monthly Volume 67.djvu/65

Rh more perfect than the requirements demand. The almost exact resemblance between the right and left sides of the body, while advantageous up to a certain point, is often far more perfect than competition requires. In fact we find the Darwinians often dodging this consideration, and referring the results to 'the laws of growth' etc. But if these 'laws of growth' exist why may they not have also carried the perfection of any organ far beyond the point at which the test of survival stops?

We meet with a somewhat similar case in the distribution of color over the bodies of animals. Granting that in some cases the presence of colors, after they have appeared, may be of some use to the animal or plant, yet the wonderful symmetry of distribution, and the gradual shading of the colors is often far more regular than appears to be required. This result also will no doubt be ascribed to 'the laws of growth' but if we once admit such a principle at work, why bring in any other outside law to explain the perfection of those characters that are useful to the organism? If when a new species appears its colors happen to be so distributed that the individual gains some advantage, so much the better! If the color does not count one way or the other, then it does not enter into the problem of survival; but if it exposes an animal to a greater risk than it can surmount, that species will fail to hold its own. The same regularity and graduation of color exists also in animals that are microscopic, and no one thinks of accounting for these conditions through selection. Why then do we need a special explanation when the animals are so large that they attract our attention?

A number of writers, of whom Eimer is perhaps the most prominent, have insisted that evolution progresses along certain definite lines that are quite independent of selection of individual variations. The process has been called orthogenesis. Certain aspects of this view are in full accord with the theory of the survival of definite variations; for, we find, in fact, one of the most characteristic features in the appearance of definite variations to be that the same forms appear over and over again, showing a definite tendency to vary in certain definite directions. A striking case of this sort is that of the japanned peacocks described by Darwin, and of the mutations of the evening primrose described by de Vries. If future work can show that a change in a given direction is likely to be followed by others in the same direction, amongst some at least of the offspring, the process will have much in common with the process of orthogenesis. If this process should be in the direction of making some particular organ more perfect than the conditions demand, the new type may persist along with the parent form also, which it need not replace. If, on the contrary, the new acquirement unfits the new species for its environment the new species