The difficulty, of course, has not escaped Weismann. His explanation depends on some conception of the potentialities of conjugation and intercrossing which I confess I cannot understand. He finds the key to the mystery in the mingling and constant recombination of determinants from different individuals produced by promiscuous intercrossing. “It is only through amphimixis [conjugation] that simultaneous harmonious adaptation of many parts becomes possible.”[71] But surely this continual mingling and recombination would, _primâ facie_, be just as likely to break up co-adaptations already forming as to give rise to new ones? Amphimixis, as we have seen, is one of the most potent forces against which the evolution of a new species has to contend. Evolution has to make head against the constant tendency of intercrossing to obliterate individual distinctions. True, if parents exhibiting the same heritable variation unite, their offspring will have that variation in a strongly marked form, and will transmit it further. But this, to be of value for evolution, presupposes the same variation occurring simultaneously in a number of individuals within reach of each other. Weismann had indeed good reason to ascribe to the action of intercrossing “a wealth and diversity of organic architecture otherwise unattainable,” but were it not supplemented by an architectural instinct of nature, the only architecture attainable would be that of the child when it empties its bricks on the floor. Consider the theory of germinal selection in the light of the following very curious case.[72] Most people have seen an example of the kind of spectacles having what are called bifocal lenses. Each lens is divided across the centre, and the focal lengths of the upper and the lower halves are different. They are intended for persons who see indistinctly both at near and at far distances—the upper half of the lens is used for looking at distant objects and the lower for reading, etc., so as to avoid the inconvenience of having a different pair of glasses for each requirement. Now there is a fish, named _Anableps_ (the Uplooker), living in estuaries on the east coast of South America which actually has its eye-lenses constructed on this principle. The pupil of the eye is divided laterally by prolongations from the iris. The significance of this extraordinary arrangement is that the fish is in the habit of swimming near the surface, and often has its eyes wholly or partly out of water, presumably to look out for attacks from birds of prey. The upper half of the eye has become adapted for vision in the air and the lower for vision in the water.