Were evolution to depend on the occurrence, by pure chance, of a few appropriate variations among a vast multitude of indifferent or disadvantageous ones, is it conceivable that we should find in nature anything like the infinite wealth of closely and beautifully adapted structure which is actually present? In particular, how are we to account for the cases in which a number of parts are so modified as to work together in harmonious co-adaptation? Each of these parts, according to Weismann, originates quite independently of the others. Take the case of the Indian leaf-butterfly already referred to.[70] The first beginnings of the midrib on Weismann’s theory had nothing to do with the rest of the rib, nor had any of the veinings with this, or with one another; and the contour of the leaf, sending out a little projection like a stalk _exactly where the midrib starts_, originated quite independently of that marking, and equally so of the leaf it mimics! To explain co-adaptations like this on Weismann’s theory is really much the same as to suppose that a picture could be painted by simply plastering the scrapings of a palette on a canvas, if only one continued the process long enough. And the marvel in question, the co-adaptation of various parts, has not been attained once or twice but, to a greater or less degree, in _every_ organism possessing any structural complexity.
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.