56

THE SHELLS OF FORAMINIFERA.

metrical forms, which often present angles and angular figures; as soon as structure is acquired, it appears as if the free mobility of the particles of living matter were interfered with, so that no similar phenomena occur amongst the higher forms of life.

The forms of the shells of the Foraminifera chiefly depend upon the manner in which the protoplasmic masses which they contain undergo multiplication; and this is probably determined by their molecular condition. The ridges on the surface of the shells, however, may result from the formation of nodal lines upon the moving mass of living matter which they contain: these are apparently always parallel to the axis of growth, even in the most distinct forms in which they occur; so that they are probably determined, like it, by the molecular condition of the protoplasm.

The forms of the shells of the Foraminifera vary within certain limits, so that many of the genera are highly polymorphic; this is probably due to the variability of their conditions of life, chiefly to rapidity of growth and the amount of food which each individual obtains. Although many existing forms are apparently identical with those of the chalk era, they still vary in the same manner : this is a strong argument against the view that

THE PERSISTENCE OF RUDIMENTS.

57

mere repetition of a character through many successive generations serves to render it more fixed. On the view of inheritance advocated in these pages, it is easy to see that mere repetition would in itself have no power to prevent subsequent deviation. Mr. Darwin has sought carefully for evidence in favour of the opinion that characters become more fixed by long inheri tance; but the conclusion he arrives at is against this view.

The persistence of rudiments after they have ceased to be of functional importance has been used as an argument both in favour of and against the hypothesis of evolution. It is evidence in favour so far as it shows the extreme probability of a genetic relation between living organisms. It has been advanced as an argument on the other side, because it is said that rudiments ought to disappear under the same influences which caused the reduction or rudimentary condition of the original organs. Under the view of inheritance here advocated, however, it is easy to see that such would not necessarily be the case. Variation is limited in every case; and, as Mr. Galton has shown, great deviations from the average are less common than slight deviations, and occur equally on either side of the mean or normal condition. Under these circumstances the develop

ment of a part must be not only useless but absolutely prejudicial before its disappearance can be determined. It is true that atrophy follows disuse; but there is no evidence to show that a part ever undergoes total absorption from this cause; and therefore we should not expect the disappearance of a rudimentary part to be determined in this manner. The nutrition of rudiments is scarcely likely to affect the nutrition of the whole organism appreciably; and all the more highly organized forms of life, even when growing rapidly, hold a large reserve of nutrient material in the circulating fluid.

We may conclude, therefore, that variation is limited by the limitation of conditions; it is further limited by the acquirement of structure; and, lastly, the action of natural selection under unchanged conditions tends to produce rigidity. Even rudimentary organs remain unless there is a direct influence at work to produce their destruction: it is not sufficient that they are useless; they must be positively harmful.

The offspring, therefore, remains like its parent, and is said to inherit the characters of its parent. It inherits these characters more closely as the complexity of the organism increases, except when conditions occur which are favourable to the development of plasticity. The more complex

laws of inheritance in its connexion with development and the phenomena of reversion remain for our consideration; these may be most readily conceived by the study of Mr. Darwin's hypothesis of Pangenesis.

MR. DARWIN'S hypothesis of Pangenesis*, like the undulatory theory of light, is one that is rendered extremely probable, although it is not proved to be true, because we are able to account for the phenomena by its application. It belongs to that class of hypotheses which contain an assumption, at present at least, incapable of proof. Dr. Hooker said of this hypothesis in 1868 †, "there is no question but that to Mr. Darwin's enunciation of the doctrine of Pangenesis we owe it that we have the clearest and most systematic résumé of the many wonderful phenomena of reproduction and inheritance that has yet appeared; and against the guarded entertainment of the hypothesis, or speculation if you will, as a means of correlating these phenomena, nothing can be urged in the present state of science."

Mr. Darwin supposes that every cell or minute fragment of an organism consists of myriads of

* Animals and Plants under Domestication, chap. xxvii. † Dr. Hooker's presidential address to the British Association at Norwich, 1868.