Parallel Paths: A Study in Biology, Ethics, and Art

by T. W. Rolleston

To continue Reinke’s explanation: Two different classes of dominants are to be recognized. These are the operative and the formative. The former control principally the chemical activities of the organism, as when a plant turns inorganic substances into sugar, albumen, etc.; the latter are the invisible architects in the organism who control its form and structure. Both are heritable, and are capable of modification within certain limits. Closely bound up with matter and energy, they are neither matter nor energy. They can be indefinitely multiplied and (to all appearance) totally destroyed. Their multiplication does not abstract energy from other known sources, nor does their destruction restore it; they do not therefore come (visibly) under the law of the conservation of energy. They operate entirely within the framework of natural laws, and can only utilize what energies are available for them at the given time and place. Every cell has its dominants; and as an organism is a synthesis, not a mere aggregate, of cells, so its individual dominant is a synthesis of the dominants of its parts. The evolution of species, like the development of an embryo, is under the control of dominants. The conditions under which they work for this end are material and physical; these conditions can, to a great extent, be ascertained and defined, but the driving force lies beyond scientific analysis.

Such is the conception of Reinke; and taken as he presents it, that is to say, merely as a kind of working hypothesis, as a means of making intelligible a vast and various mass of phenomena, it seems admirably suited to its purpose. It remains to add, though Reinke himself does not say so, that this conception of the dominants appears to harmonize remarkably with what has been put forward in regard to cell-structure and reproduction. The chromosomes are probably the material vehicles of the dominants; in fact, Weismann’s determinants seem to be the same thing under another name, though Weismann conceives them rather from the point of view of the scientist, and Reinke from that of the metaphysician.

We have now arrived at an intellectual conception under which to range the phenomena (not the ultimate nature) of vital response. Let us apply it to the question of evolution. The following passage from Henslow’s Origin of Plant Structures[91] may serve to introduce this part of our discussion:—

“The question ... resolves itself into this: which probability or hypothesis do the facts of the case seem to favour most, viz. that indefinite variations arise from some assumed internal causes, of which variations only those in harmony with the environment survive, and are said, therefore, metaphorically, to be selected by it; or is it that the external forces of the environment excite the variability which is inherent in plants, and call into action the responsive power of the protoplasm in the various species of plants, which thus all tend to put on the same, or similar, or at least adaptive and definite variations of one sort or another, so that there are no indiscriminate or wasted variations[92] at all? I know an abundance of facts which support the latter contention, but none whatever in illustration of the former hypothesis.”

Here is the action of the dominants in evolution placed in the clearest light. To prove the truth of Professor Henslow’s contention it is necessary not only to study organisms in situations where they have been established for many generations or centuries, but to see how they behave on transportation to a new kind of environment. The cases which can be adduced are numerous and convincing. Thus Mr. D. Dewar reported to Mr. Henslow that on introducing at Kew a cress, Arabis anachortica, found in cave-like situations in the Alps, and having very thin, papery leaves, it turned, when raised from seed, into a different species, Arabis alpina. The change took only three generations to accomplish.[93]

Bulbous roots have it among their functions to store up moisture for the plant they belong to. Haeckel has shown that the grass Poa bulbosa, on being cultivated in moist soil, almost lost its bulbous character. Contrariwise we find that many plants not bulbous elsewhere are observed to be so when growing on the dry Karoo in South Africa.[94]

Spines on a plant are usual accompaniments of dryness in soil or atmosphere. Ononis spinosa has an excessively spiny variety, termed horrida, which is found on maritime sands. Grown in very rich moist situations, it gradually loses its spines and they ultimately disappear entirely.[95]

In the animal world experimental cultivation is not at all so easy, but the facts observed all go to support the view that the response to environment is direct and definite. The small shrimp-like crustacean, Artemia salina is a case frequently quoted. It lives in salt pools by the Black Sea, and it has been found that by breeding it in water of which the salinity is gradually decreased, the creature in a few generations assumes a type commonly assigned not merely to a different species but to a different genus—Branchipus stagnalis.[96]

Perhaps the most remarkable instance of a transformation produced by the influence of environment is that of the Mexican water-newt, Axolotl. When gradually accustomed to live on dry land, this creature usually throws off its gills, develops lungs, alters the shape of its tail, and takes on all the characteristics of a terrestrial instead of an aquatic reptile. This transformation does not take generations to accomplish—it happens in one individual in the course of a few weeks or months. When found in the terrestrial form, the Axolotl is called Amblystoma tigrinum, and is classed among the salamanders. Its progeny are then Amblystomas, and they do not naturally revert to the Axolotl type, although under certain circumstances the steps of this amazing transformation can be retraced. The Axolotl is not a larva in the ordinary sense of the word, for it is not an imperfect creature; it is sexually mature, and in most cases, in nature, probably never develops into an Amblystoma, nor do the progeny of the Amblystomas begin as Axolotls. What we have here is probably, as Weismann plausibly suggests, a case of a species which has almost reached the stage of evolution from an aquatic into a terrestrial form, so that a sufficient impulse from its environment suffices to send it over the border. Internal forces have evidently prepared the way for the change, and the process does not in the least resemble the mechanical selection of suitable characters from a crowd of fortuitous variations.[97]

The case of the Porto Santo rabbit may also be quoted in this connexion. In the year 1419 the young born of a tame Spanish rabbit were put ashore on the island of Porto Santo near Madeira. No rabbits then existed on the island. They have since increased enormously, and have quite changed their appearance. They have acquired a peculiar colour, are very small, rat-like in shape, have nocturnal habits, and are noted for their extreme wildness. They no longer pair with the European rabbit. The case was observed by Haeckel, who styled the new species Lepus Huxleyi.[98]