Page:Popular Science Monthly Volume 82.djvu/398

394 Darwin and Wallace have talked of the accumulation of small variations, of the effects of natural selection on the perpetual minute variations which all species exhibit. Antiselectionists answer that most of these are non-heritable "fluctuating variations," and as for the rest, they are usually not small, nor is the variation "continuous." Moreover, selection soon gets to the end of its rope. So it seems when we are looking at a single pair, or two or three pairs of Mendelian allelomorphs, all active independently of one another. This, however, is not a true picture of living animals, which are bundles of uncounted "factors," acting together in various ways. Any single factor depends for its appearance and form of manifestation on all the others, as Wilson has urged. It is not a thing by itself; it is the result of a complex equation. Sometimes we are getting along well enough with our experiments, when suddenly things go wrong; not because of error in our theory, but because some new factor, which we were not watching, has come in and disturbed the results. Thus, in breeding red sunflowers, we predicted, and got, a dark red homozygous flower. We also got, but did not predict, a homozygous red in which only the basal half of each ray was of that color. The fact is that many of the wild sunflowers carry a factor for marking, which can be seen with difficulty on close inspection, but in the red it comes out strongly. For reasons of this sort we have not only the complications due to the multitude of factors or determiners, but also those caused by their interactions. Inasmuch as they may influence each other strongly or slightly, and in all sorts of different ways, the net result is that in the more complex types of life we have almost infinite possibilities of variation, quite aside from any question of the alteration of the determiners themselves. Add to this the complications due (it appears) to accidents in the maturation process of the germ cells—such an "accident" probably gave rise to the red sunflower—and we have in most cases as much material for natural selection to operate upon as Darwin or Wallace ever postulated. Enough, indeed, to account for all the wonderful adaptations in the tropical fauna and flora, when we consider the time available for their production.

It has recently been announced, as the result of the museum work of Dr. K. Jordan, combined with the field and breeding observations of Dr. G. D. H. Carpenter, that an African butterfly of the genus Pseudacræa occurs in a variety of forms, which imitate species of Planema flying with it. The extraordinary thing is that one phase of this Pseudacræa is sexually dimorphic, imitating a dimorphic Planema, while in the same forests it also occurs in two monomorphic forms, resembling two other monomorphic species of Planema. Dr. Carpenter succeeded in breeding one of the monomorphic Pseudacræas from an