The American Naturalist 32(374): 103-104 (Feb 1898)
Some Theories of Heredity and of the Origin of Species Considered in Relation to the Phenomena of Hybridization.
Walter T. Swingle

Owing to limited time, the speaker treated only the first portion of his theme, viz., the bearing of the facts of hybridization on some theories of heredity. It was pointed out that Weismann's theory of reduction of chromosomes, though giving a plausible explanation of the differences observed between the first (uniform) and second (polymorphic) generations of most hybrids, is not in accord with the observed phenomena of spore and pollen formation in higher plants, and, moreover, fails to account for the extreme polymorphism often observed in the first generation of hybrids of races of cultivated plants or closely related species, as, for example, some racial hybrids of maize and some specific hybrids of Lychnis and Digitalis. Mr. Swingle considered it necessary to assume in some such cases, at least, a predetermination of the characters of the hybrid at the time of fusion of the male and female nuclei.

Since the male and female chromosomes probably persist side by side unchanged in number, and possibly unchanged in quality during the whole of the ontogeny of the hybrid (reduction not occurring until the close of the first generation), it is therefore necessary to assume, in order to explain the observed fact of divergence of character in the first generation of some hybrids, that the influence exerted during ontogeny of the hybrid by the material bearers of heredity is, at least in some cases, a function of their relative positions, and, further, that in most cases the relative positions of these bearers of heredity, as determined at the moment of fusion of the male and female nuclei, would persist unchanged throughout ontogeny of the offspring. Some exceptional cases, such as reversions to the one or the other parent form of a larger or smaller part of the hybrid, would be explained by assuming some change in the disposition of the units of hereditary substance, whereby they assumed a new position of partial or complete stability. It was suggested that possibly the difference between uniform and polymorphic hybrids of the first generation is due to a more complete intermingling of the hereditary particles in case of polymorphic hybrids (offspring of closely related organisms), whereby many differing combinations would be possible, and, in case of uniform hybrids (mostly offspring of distinct species or very different races of the same species), to greater or less aversion to commingling between the two more diverse sorts of particles, whereby but one uniform and stable configuration would result, allowing both sorts of hereditary substance to act equally.

Xenia, or the communication of the paternal characters to parts of the mother plant in the immediate neighborhood of the developing embryo, was held to be well established in case of some races of maize by the work of Dudley, Savi, de Vilmorin, Hildebrand, Kornicke, Sturtevant, Burrill, Kellerman and Swingle, McCluer, Tracy, Hays, and others, and in case of some races of peas by the work of Wiegmann, Gartner, Berkeley, Laxton, and Darwin. The converse phenomena of the mother plant influencing the characters of the developing embryo is occasionally reported; for instance, in hybrids of Digitalis, by Gartner, and in hybrids of Nymphaea, by Caspary.

These phenomena are inexplicable by the current theories of heredity, and perhaps in consequence have been neglected. They necessitate the assumption that hereditary influences can be transported from cell to cell for some distance. It was suggested that this transport may occur either along the intercellular filaments which pass through the walls, or by means of diffusible substances capable of acting on the hereditary particles of distant cells. Townsend's proof of the conduction of the stimulus which results in wall formation over long, slender threads of protoplasm in plasmolyzed cells may be considered as hinting the possibility of the former explanation, while Beijerinck's claim that the developing larvae of some gall insects secrete substances which diffuse into and control the ontogeny of neighboring meristematic or partially developed tissue cells of the host plant foreshadows the latter hypothesis.