Mr. Henslow adduces observations and experiments by Buckman, Bailey, Lesage, Lothelier, Costantin, Bonnier, and others, all demonstrating that the environment acts directly on the plant.
Henslow also suggests that endogens have originated from exogenous plants through self-adaptation to an aquatic habit,[256] which is in line with our idea that certain classes of animals have diverged from the more primitive ones by change of habit, although this has led to the development of new class-characteristics by use and disuse, phenomena which naturally do not operate in plants, owing to their fixed conditions.
Other botanists—French, German, and English—have also been led to believe in the direct influence of the milieu, or environment. Such are Viet,[257] and Scott Elliot,[258] who attributes the growth of bulbs to the “direct influence of the climate.”
In a recent work Costantin[259] shares the belief emphatically held by some German botanists in the direct influence of the environment not only as modifying the form, but also as impressing, without the aid of natural selection, that form on the species or part of its inherited stock; and one chapter is devoted to an attempt to establish the thesis that acquired characters are inherited.
In his essay “On Dynamic Influences in Evolution” W. H. Dall[260] holds the view that—
“The environment stands in a relation to the individual such as the hammer and anvil bear to the blacksmith’s hot iron. The organism suffers during its entire existence a continuous series of mechanical impacts, none the less real because invisible, or disguised by the fact that some of them are precipitated by voluntary effort of the individual itself.... It is probable that since the initiation of life upon the planet no two organisms have ever been subjected to exactly the same dynamic influences during their development.... The reactions of the organism against the physical forces and mechanical properties of its environment are abundantly sufficient, if we are granted a single organism, with a tendency to grow, to begin with; time for the operation of the forces; and the principle of the survival of the fittest.”
In his paper on the hinge of Pelecypod molluscs and its development, he has pointed out a number of the particular ways in which the dynamics of the environment may act on the characters of the hinge and shell of bivalve molluscs. He has also shown that the initiation and development of the columellar plaits in Voluta, Mitra, and other gasteropod molluscs “are the necessary mechanical result of certain comparatively simple physical conditions; and that the variations and peculiarities connected with these plaits perfectly harmonize with the results which follow within organic material subjected to analogous stresses.”
In the same line of study is Dr. R. T. Jackson’s[261] work on the mechanical origin of characters in the lamellibranch molluscs. “The bivalve nature of the shell doubtless arose,” he says, “from the splitting on the median line of a primitive univalvular ancestor;” and he adds: “A parallel case is seen in the development of a bivalve shell in ancient crustaceans;” in both types of shells “the form is induced by the mechanical conditions of the case.” The adductor muscles of bivalve molluscs and crustaceans are, he shows plainly, the necessary consequence of the bivalvular condition.
In his theory as to the origin of the siphon of the clam (Mya arenaria), he explains it in a manner identical with Lamarck’s explanations of the origin of the wading and swimming birds, etc., even to the use of the words “effort” and “habit.”
“In Mya arenaria we find a highly elongated siphon. In the young the siphon hardly extends beyond the borders of the valves, and then the animal lives at or close to the surface. In progressive growth, as the animal burrows deeper, the siphon elongates, until it attains a length many times the total length of the valves.
“The ontogeny of the individual and the paleontology of the family both show that Mya came from a form with a very abbreviated siphon, and it seems evident that the long siphon of this genus was brought about by the effort to reach the surface induced by the habit of deep burial.”
“The tendency to equalize the form of growth in a horizontal plane, or the geomalic tendency of Professor Hyatt,[262] is seen markedly in pelecypods. In forms which crawl on the free borders of the valves, the right and left growth in relation to the perpendicular is obvious, and agrees with the right and left sides of the animal. In Pecten the animal at rest lies on the right valve, and swims or flies with the right valve lowermost. Here equalization to the right and left of the perpendicular line passing through the centre of gravity is very marked (especially in the Vola division of the group); but the induced right and left aspect corresponds to the dorsal and ventral sides of the animal, not the right and left sides, as in the former case. Lima, a near ally of Pecten, swims with the edges of the valves perpendicular. In this case the geomalic growth corresponds to the right and left sides of the animal.
“The oyster has a deep or spoon-shaped attached valve, and a flat or flatter free valve. This form, or a modification of it, we find to be characteristic of all pelecypods which are attached to a foreign object of support by the cementation of one valve. All are highly modified, and are strikingly different from the normal form seen in locomotive types of the group. The oyster may be taken as the type of the form adopted by attached pelecypods. The two valves are unequal, the attached valve being concave, the free valve flat; but they are not only unequal, they are often very dissimilar—as different as if they belonged to a distinct type in what would be considered typical forms. This is remarkable as a case of acquired and inherited characteristics finding very different expression in the two valves of a group belonging to a class typically equivalvular. The attached valve is the most highly modified, and the free is least modified, retaining more fully ancestral characters. Therefore, it is to the free young before fixation takes place and to the free, least-modified valve that we must turn in tracing genetic relations of attached groups. Another characteristic of attached pelecypods is camerated structure, which is most frequent and extensive in the thick attached valve. The form as above described is characteristic of the Ostreidæ, Hinnites, Spondylus, and Plicatula, Dimya, Pernostrea, Aetheria, and Mulleria; and Chama and its near allies. These various genera, though ostreiform in the adult, are equivalvular and of totally different form in the free young. The several types cited are from widely separated families of pelecypods, yet all, under the same given conditions, adopt a closely similar form, which is strong proof that common forces acting on all alike have induced the resulting form. What the forces are that have induced this form it is not easy to see from the study of this form alone; but the ostrean form is the base of a series, from the summit of which we get a clearer view.” (Amer. Nat., pp. 18–20.)