The same close correlation between the sexual organs and the other parts of the body occurs among plants as generally as among animals. If one wishes to obtain an abundance of fruit from a garden plant, the growth of the leaves is curtailed by cutting off some of them. If, on the other hand, an ornamental plant with a luxuriance of large and beautiful leaves is desired, then the development of the blossoms and fruit is prevented by cutting off the flower buds. In both cases one system of organs develops at the cost of the others. Thus, also, most variations in the formation of leaves in wild plants result in corresponding transformations of the generative parts or blossoms. The great importance of this "compensation of development," of this "correlation of parts," has been already set forth by Goethe, by Geoffroy St. Hilaire, and other nature-philosophers. It rests mainly upon the fact that direct or actual adaptation cannot produce an important change in a single part of the body, without at the same time affecting the whole organism.
The correlative adaptation between the reproductive organs and the other parts of the body deserves a very special consideration, because it is, above all others, likely to throw light upon the obscure and mysterious phenomena of indirect or potential adaptation, which have already been considered. For just as every change of the sexual organs powerfully reacts upon the rest of the body, so on the other hand every important change in another part of the body must necessarily more or less react on the sexual organs. This reaction, however, will only become perceptible in the formation of the offspring which arise out of the changed generative parts. It is, in fact, precisely those remarkable and imperceptible changes of the genital system (in themselves utterly insignificant changes)--changes of the eggs and the sperm--brought about by such correlations, which have the greatest influence upon the formation of the offspring, and all the phenomena of indirect or potential adaptation previously mentioned may in the end be traced to correlative adaptation.
A further series of remarkable examples of correlative adaptation is furnished by the different animals and plants which become degenerated through parasitic life or parasitism. No other change in the mode of life so much affects the shapes of organisms as the adoption of a parasitical life. Plants thereby lose their green leaves; as, for instance, our native parasitical plants, Orobanche, Lathræa, Monotropa. Animals which originally have lived freely and independently, but afterwards adopt a parasitical mode of life on other animals or plants, in the first place cease to use their organs of motion and their organs of sense. The loss of this activity is succeeded by the loss of the organs themselves, and thus we find, for example, many crabs, or crustacea, which in their youth possess a tolerably high degree of organization, viz., legs, antennæ, and eyes, in old age completely degenerate, living as parasites, without eyes, without apparatus of motion, and without antennæ. The lively, active form of youth, has become a shapeless, motionless lump. Only the most necessary organs of nutrition and propagation retain their activity; all the rest of the body has degenerated. Evidently these complete transformations are, to a large extent, the direct consequences of cumulative adaption, of the non-use and defective exercise of the organs, but a great portion of them must certainly be attributed also to correlative adaptation. (Compare Plate X. and XI.)