CHAPTER II

CAN RUDIMENTARY INSTITUTIONS OR ORGANS RESUME

THEIR PRIMITIVE FUNCTIONS?

THERE is no break of continuity between a rudimentary organ and the complete ancestral organ: the rudimentary organ is the ancestral organ transmitted by inheritance. It is the same organ not only because it has the same form, but because it is actually a part of it. For the reduced organ to return to its ancestral condition and resume its ancestral function, it must retrace the series of steps in degeneration along which it has passed.

So also in sociology there are many instances of institutions which are rudimentary, but which retain their original form because of uninterrupted imitative transmission. In their case also resumption of the primitive functional activity would imply a retracing of the degenerate steps.

This necessity of retracing shows at once that after a certain amount of degeneration resumption becomes impossible. We shall find in the few cases we are able to adduce, that when organs retrace their steps and resume their ancestral functions, degeneration had not gone very far in them.

I. Animals.

SECTION I.

Rudimentary organs.

Among animals it is very unusual for a rudimentary organ to become active again.

The only cases show that in them no great amount of degeneration had taken place.

1. Muscles of the ear in man.- It is known that the human ear possesses a number of intrinsic and extrinsic muscles reduced to delicate fibres, and incapable of producing movement of the whole ear or of one part of the ear on another part. In some abnormal persons, however, certain of these muscles may be well developed, making movements of the ear possible.

2. The abdomen and appendages in deep-sea hermit-crabs.-The hermit-crabs of the deep sea are another instance of reversion to an ancestral form. Littoral hermit-crabs inhabit the spiral shells of Gastropods, and to suit this mode of life the body is unsymmetrical, the appendages of one side being rudimentary. In the depths of the ocean such spiral shells are rare, and the crabs either abandon this mode of life or live in straighter shells. In consequence the limbs and the abdomen become nearly symmetrical again. It is plain, however, that in the littoral crabs these structures are not truly rudimentary.

II. Plants. In plants it is very difficult to distinguish between the reappearance of lost organs and the formation of new organs.

1. Hermaphrodite flowers in Melandryum.-The Hermaphrodite flowers of melandryum (fig. 59) may be flowers which after being unisexual have again become hermaphrodite, or they may have retained

the primitive type. We cannot decide between the alternatives.

2. Branches of Colletia cruciata, Crataegus, Vicia Faba, etc.- Some cases, however, point clearly to a renewed development of rudimentary organs. Here are some examples. (See also, further on, page 244 on hybrid individuals of Pentstemon.)

Colletia cruciata (fig. 72) in the normal adult condition bears large flattened branches, which serve for assimilation and possess only very rudimentary leaves. Sometimes, however, the plant may give rise to more slender branches with normal assimilating leaves. These branches and leaves are probably the reappearance of the ancestral condition.

Wild pear and apple trees produce small lateral branches which are transformed into spines. These thorns have evidently arisen from normal lateral branches which originally bore leaves. In the cultivated varieties these lateral branches have resumed the leaf-bearing habit.

In the hawthorn (Crataegus) the lateral branches are similarly modified into spines. None the less, while these spines are still young they may be artificially stimulated to produce leaves by cutting the principal stem.

The branches of Vicia faba bear low down a set of rudimentary leaves. If the main stem be lopped while still quite young, the usually rudimentary leaves grow to the normal size.1

1 Goebel, Beiträge zur Morphologie und Physiologie des Blattes. Bot. Zeit., 1880.

A

B

FIG. 72.-Colletia cruciata (after Goebel, Pflanzenbiologische Schilderungen vol. i., p. 17). A, norinal branch. B, branch recurring to ancestral form; (nasural size).

Many plants such as Sempervivum 1 (fig. 73) normally possess a very much shortened stem. However, if the plant be grown in a saturated atmosphere the internodes of the stem lengthen out.

FIG. 73.-Sempervivum tectorum.

A (left figure), normal branch; B (right figure), banch grown for several months in a saturated atmosphere.

Certain Veronicas (fig. 74) bear only small scaly leaves. Cultivation of these plants in an atmosphere saturated with water, results in the appearance of normal leaves.

Lastly, we may quote again the instances given by Goebel of the production by Equisetum arvense of normal leaves under special conditions."

1 The branch figured here was grown by G. Clautriau, in the Brussels Botanical Institute.

2 Goebel, Ueber die Fruchtsprosse der Equiseten. (Ber. d. d. Bot. Ges.), vol. iv. p. 184, 1886.