Imágenes de páginas
PDF
EPUB

CILIA AND FLAGELLA.

31

the acquirement of an external hardened covering. Their reticulation could no longer take place, and the appendage would either become simple or permanently branched. Its power of movement would probably become concentrated and definite; and either a cilium or flagellum would result when the protruded portion became simple, or such an organ as the lophophore of a polyzoan with its ciliated appendages when it remained compound.

Such changes as these would certainly account for the formation of the flagellate type of infusorian from the reticulose rhizopod; and these considerations, as we have seen, afford indications of the mode in which far more complex structures have originated.

Another similar series of changes resulting from the gradual hardening of the body-wall of the Rhizopoda would lead to the formation of many points of motion on the surface. We see the steps of such a process in the perforate Foraminifera, where numerous pseudopodia are protruded from various parts of the body-mass, in the sun-animalcule, Actinophrys, where the pseudopodia are stiff and circumscribed by the hard external body-substance, and in many other Radiolarians (Plate III. figs. 1, 2, and 3). We are thus led to the ciliated Infusoria by an easy transition.

Future observations directed to these points will

32

RECAPITULATION.

1

probably throw light upon this important subject ; at present, however, it may be safely held that the energy set free in living matter may either produce general movements of the mass or special movements of portions of it; and as the movements become more circumscribed, they become more manifest and more rapid.

The gradual acquirement of such organs as cilia, and of other forms of structure, the nature of which will be considered in another chapter, is clearly guided and regulated; otherwise it could not have progressed in the definite and regular manner in which it has in nature, and organisms could not have attained the perfect adaptation to surrounding circumstances which they exhibit.

The laws which have so guided and regulated the evolution of the animal and vegetable kingdoms are doubtless extremely complex; but one of the most powerful, perhaps the most powerful, agent of evolution is the principle of Natural Selection, which may be fitly named, after its great discoverer and expounder, Darwin's Law.

[merged small][ocr errors][merged small][merged small]

NATURAL SELECTION “ signifies the preservation of such variations* as arise and are beneficial to the being under its conditions of life” t. Mr. Darwin shows that variation exists in nature : every animal and plant undoubtedly presents individual differences. These variations are larger in some species than in others, and frequently merge so gradually and imperceptibly into the characters of a so-called variety that it is sometimes impossible to determine to which stock an individual belongs. This is well seen in polymorphic genera, such as brambles, Rubus (Plates I. & II.), and gum-trees, Eucalyptus, amongst plants, and certain beetles, Harpalus, and some genera of mollusks, Brachiopods, amongst animals. Polymorphism occurs more rarely, perhaps, amongst the more highly differentiated forms of life; but certain genera of monkeys are undoubtedly

* The term variation is used in this essay to express the deviation of an individual from its parent form; the term variety to express a greater deviation, believed by the author to be the result of accumulated variations. † Darwin, Origin of Species,' p. 63.

[ocr errors][merged small]

polymorphic. In the American genus Cebus the. various forms are ranked by some naturalists as species, by others as mere geographical races *.

Individual differences have undoubtedly a certain influence on the life of the animal or plant ; this is shown by Mr. Darwin in numerous examples. The poisonous action of the paint-root (Lachnanthes) on white pigs, and its innocuous influence on black pigs, in Virginia, is a striking instance of this kind. It is impossible to say how slight a variation may determine the life or death of an individual in the struggle for existence and constant competition to obtain food. A slight difference of shade in the colour of a creature may render it inconspicuous, and enable it to evade its enemies; and, as we have seen, this may also correspond to altered constitution, so that it is enabled to feed on material which would otherwise have a more or less baneful influence upon it. A slight increase of strength or rapidity of growth may be of immense importance in its competition with allied forms; increased fleetness, or the special development of some organ to a slightly greater degree than in kindred species, may be of paramount importance. Such and similar advantages would certainly in many cases enable a possessor to survive in the struggle for life. Mr. Darwin shows how man has been able to

* Darwin, ' Descent of Man,' vol. i. p. 227.

VARIATION AND INHERITANCE.

35

modify the horse, the dog, and the pigeon by selecting certain individuals to breed from, and justly infers that, since man has been able in a short time to produce very marked varieties by the gradual accumulation of selected variations, the much more accurate action of natural selection, which results from the agency of external conditions acting through immense periods of time has produced far greater results. The gradual accumulation of variations, whether under natural or artificial selection, is the result of inheritance or transmission to the offspring.

In order to understand fully the effect of Darwin's Law on evolution, it will be necessary to consider the laws of variation and inheritance as far as possible with our present knowledge. Variation and inheritance are undoubtedly complemental to each other, just as resistance and conductivity are in an electrical conductor: to use a mathematical expression, they are inversely proportional to each other.

As we have already seen, extreme susceptibility to undergo change (or, in other words, to vary) is the principal characteristic by which protoplasm is distinguished from inorganic matter. The power of transmitting characters truly, or without variation, is so diametrically opposed to this property, that, although hereditary transmission is so universal and powerful in its action, it must, if evolution be ad-'

[ocr errors]
« AnteriorContinuar »