We have another striking example among the invertebrates in the snails (gasteropoda). The great majority of these mollusks are characterized by the spiral shape of their shells. This variously shaped, and often prettily colored and marked, snail's house is in essence a spirally coiled tube, closed at the upper end and open at the lower (or mouth): the mollusk can at any moment withdraw into its tube. The comparative anatomy and ontogeny of the snails teach us that this spiral shell came originally from a simple discoid or cylindrical dorsal covering of the once bilateral-symmetrical mollusk, by the two sides of the body having an unequal growth. The cause of it was a purely mechanical factor—the sinking of the growing visceral sac, covered with the shell, to one side; one part of the viscera contained in it (the heart, kidneys, liver, etc.) grew more strongly on one side than the other in consequence of this; and this was accompanied by considerable displacement and modification of the neighboring parts, especially the gills. In most snails one of the gills and kidneys and the ventricle of the heart corresponding to these have disappeared altogether, only those of the opposite side remaining; and the latter have moved from the right side to the left, or vice versa. The conspicuous lack of symmetry between the two halves of the body which resulted from this finds expression in the spiral form of the snail's shell. This remarkable ontogenetic metamorphosis also can be fully explained by a corresponding phylogenetic process, and affords a very fine instance of the inheritance of acquired characters.
There are also many examples of this asymmetry of bilateral forms in the plant world, such as the green foliage-leaves of the familiar begonia and the blooms of canna.
IV. The Centraporia.—Few organic forms are completely irregular and without axes, as usually the attraction to the earth (geotaxis) or to the nearest object determines the special direction of growth, and so the formation of an axis in some direction or other. Nevertheless, we may instance as quite irregular the soft and ever-changing plasma-bodies of many rhizopods, the amœbinæ, mycetozoa, etc. Most of the sponges also—which we regard as stocks of gastræads—are completely irregular in structure; the most familiar example is the common bath-sponge.
An impartial and thorough study of organic forms has convinced me that their actual, infinitely varied configurations may all be reduced to the few typical forms I have described. Comparative anatomy and ontogeny further teach us that the countless modifying processes which have led to the appearance of the various species have acted by adaptation to different environments, habits, and customs, and give us, in conjunction with heredity, a physiological explanation of this morphological transformation. But the question arises as to the origin of these few geometrically definable types, and the cause of their divergence.
In this important and difficult question we find a great variety of opinions and a strong leaning to dualistic and mystic theories. Educated laymen, who have only a partial and imperfect acquaintance with the biological facts, think that they are justified here in appealing to a supernatural creation of forms. They contend that only a wise creator, following a rational and conscious design, could produce such structures. Even distinguished and informed scientists lean in this matter towards mystic and transcendental ideas; they believe that the ordinary natural forces do not suffice to explain these phenomena, and that at least for the first construction of these fundamental types we must postulate a deliberate creative thought, a design, or some such teleological cause, and therefore consciously acting final causes. So say Nägeli and Alexander Braun.
In direct opposition to this, I have ever maintained the view that the action of familiar physical forces—mechanical efficient causes—fully suffices to explain the origin and transformation of these fundamental types, as well as for all other biological and inorganic processes. In order to understand this monistic position thoroughly, and to meet the errors of dualism, we must bear in mind always the radical processes of growth which control all organic and inorganic configuration, and also the long chain of advancing stages of development, which lead us from the simplest protists, the monera, to the most advanced organisms.
The unicellular organisms exhibit the greatest variety from the promorphological point of view. In the single class of the radiolaria we find all imaginable geometrical types represented. This is seen in a glance at the one hundred and forty plates on which I have depicted thousands of these graceful little protozoa in my monograph (Challenger Report, vol. xviii.). On the other hand, the monera, at the lowest stage of organic life, the structureless organisms without organs that live on the very frontier of the inorganic world, are very simple. Especially interesting in this connection are the chromacea, which have hitherto been so undeservedly and so incomprehensibly neglected. Among the well-known and widely distributed chroococcacea, the chroococcus, cœlosphærium, and aphanocapsa are quite the most primitive of all organisms known to us—and at the same time the organisms that enable us best to understand the origin of life by spontaneous generation (archigony). The whole organism is merely a tiny, bluish-green globule of plasm, without any structure, or only surrounded by a thin membrane; its fundamental form is the simplest of all, the centraxial smooth sphere. Next to these are the oscillaria and nostochina, social chromacea, which have the appearance of thin, bluish-green threads. They consist of simple primitive (unnucleated) cells joined to each other; they seem often to be flattened into a discoid shape as a result of close conjunction. Many protists are found in two conditions, one mobile with very varied and changeable forms, and one stationary with a globular shape. But when the separate living cell begins to form a firm skeleton or protective cover for itself, it may assume the most varied and often most complicated forms. In this respect the class of the radiolaria among the protozoa, and the class of the diatomes among the protophyta (both of which have flinty shells), surpass all the other groups of the diversified realm of the protists. In my Art-forms in Nature I have given a selection of their most beautiful forms (diatomes, A-f, 4, 84; radiolaria, A-f, 1, 11, 21, 22, 31, 41, 51, 61, 71, 95). The most remarkable and most important fact about them is that the artistic builders of these wonderful and often very ingenious and intricate flinty structures are merely the plastidules or micella, the molecular and microscopically invisible constituents of the soft viscous plasm (sarcode).
The configuration of the histona differs essentially from that of the protists, since in the case of the latter the simple unicellular body produces for itself alone the whole form and vital action of the organism, while in the histona this is done by the cell state, or the social combination of a number of different cells, which make up the tissue body. Hence the ideal type which we can always define in the actual histonal form has quite a different significance from that in the unicellular protists. In the latter we find the utmost diversity in the configuration of the independent living cells and the protective cover it forms; among the histona the number of fundamental forms is limited. It is true that the cells themselves which make up the tissues may exhibit a great variety in form and structure; but the number of the different tissues which they make up is small, and so is the number of ideal types exhibited by the organism they combine to form—the sprout (culmus) in the plant kingdom and the person in the animal kingdom. The same may be said of the stock (cormus) in both kingdoms—that is to say, of the higher individual unity which is constituted by the union of several sprouts or persons.
The two classes of fundamental forms which are especially found in the plant sprouts or the animal persons are the radial and bilateral. The one is determined by the stationary life, the other by free movement in a certain attitude and direction (swimming in water or creeping on the ground). Hence we find the radial form (as pyramidal) predominant in the blooms and fruits of the metaphyta, and the persons of the polyps, corals, and regular echinoderms. On the other hand, the bilateral or dorsiventral form preponderates in most free-moving animals; though it is also found in many flowers (papilionaceous and labial flowers, orchids, and others that are fertilized by insects). Here we have to seek the cause of the bilateralism in different features, in the relations with the insects, in the mode of their fastening to and distribution on the stalk (for the green foliage leaves), and so on.
The complex individuals of the first order, the stocks (cormi), are more dependent in their growth on the spatial conditions of their environment than the sprouts or persons; hence their typical form is generally more or less irregular, and rarely bilateral.