Evolution

by James A. S. Watson

Fig. 24.—Cercomonas, a form intermediate between the crawling Amœba type and the free-swimming Paramœcium type.

There is another interesting fact in connection with Paramœcium. Under natural conditions, division and redivision continue in the ordinary way for a large and indefinite number of generations. But very occasionally, a process known as conjugation occurs. Two individuals lay themselves side by side, and partially unite; they exchange portions of their nuclear substance, and finally separate again, simple division afterwards proceeding as before. Conjugation, although distinctly different from the ordinary process of sexual reproduction, appears to serve the same purpose. Until quite lately its meaning, and that of the process of sexual reproduction in general, seemed to bid fair to remain a perpetual puzzle to biologists. But at last we seem to be approaching the solution. The characters of a species are determined, it is tolerably certain, by the constitution of the cell nucleus, and accordingly as this varies from one individual to another, so the characters of the individuals will vary. Now, if simple division were to continue indefinitely, successive generations would be produced on the same plan, and the racial characters would in the main remain constant. But conditions of life vary from time to time and from place to place, and the particular type which succeeds best under one set of circumstances may be ill adapted for another. It is therefore an advantage to a race to be capable of variation. And the process of sexual reproduction, by continually bringing about a mixture of the nuclear substance, ensures the regular production of a variety of types. Of these various combinations of characters the few that are suited to the prevailing conditions will, for the time being, constitute the dominant types. When conditions change, fresh types will be available to replace them. The process of conjugation is illustrated in Fig. 25.

Fig. 25.—Stages in conjugation of Paramœcium.

meg., The meganucleus; mic., the micronucleus, which divides, and half of which is exchanged; p.b., Polar bodies, which the micronucleus throws off, and which disappear.

From Dendy's Outlines of Evolutionary Biology (Constable).

There are many groups of one-celled animals other than those typified in the Amœba and the Paramœcium, but they do not appear to have any significance so far as the descent of the higher animals is concerned, and they therefore do not immediately concern us.

We have already mentioned that water is the life medium of the slipper animalcule. It was destined to remain the natural element, both of animals and of plants, throughout many subsequent stages of progress. The reason of this is not far to seek. Active protoplasm consists to the extent of about three-fourths of water, and a plentiful supply of this is one of the essentials for the continuance of active life. Therefore, before the conquest of the dry land could be accomplished, devices had to be evolved both for maintaining and for conserving the water supply—roots in the plant; in the animal, some method of locomotion by land or air, so that water could be frequently reached; protection against evaporation, in the form of a skin, in both; and numerous other special devices. Add to this the fact that locomotion on land presents much greater difficulties than that in water, and it will hardly occasion surprise that vast ages were yet to be required before the Evolution process could produce a land animal.

A striking analogy may be drawn between animal Evolution, from this point onwards, and social Evolution. In the latter case we begin with men, brought by a slow process of Evolution to a high state of individual perfection, living in a state of savage individualism. Each thinks and acts for himself, provides his own food, raiment, and dwelling; constitutes his own standing army and police. From this condition of affairs there has gradually been developed the modern social arrangement, by which each individual helps to carry out some distinctly special part of work for the community—be it wheat-growing, cloth-weaving, bricklaying, or the arresting of burglars—and trusts to the community for his requirements in all other directions. These requirements themselves have so multiplied during the course of social Evolution that innumerable forms of activity have sprung up between those occupations which provide the original necessities of life. The essence of the whole process has been co-operation and the division of labour.

In the story of animal Evolution we have reached a point where a highly perfected individual cell has been produced, which carries out for itself, and for itself alone, all the activities of life. From now onwards, co-operation and specialisation are the watchwords of progress. There is a clubbing together, first of a few cells, then of hundreds, and finally of millions upon millions, to form the bodies corporate which we recognise as individual higher animals. Division and distribution, subdivision and further distribution of the life activities proceed at the same time, until we reach the condition prevailing in the higher animals, where the degree of specialisation almost passes conception. In such there is, to begin with, a vast frontier army of skin cells, occupied in securing peace, as far as possible, for the industries that go on within. There are directors and controllers of these industries—the brain cells—with a myriad of workers under their guidance, and a great and complex telegraph system between. The workers themselves are of all descriptions—common labourers like the cells of the muscles; transport workers like those of the circulatory system; skilled factory hands like those of the glands; even scavengers in the shape of the sweat gland and kidney cells. Nay, there is even a numerous police force, of white blood corpuscles, which patrol everywhere, arresting intruders and disposing of them by the summary method of swallowing them whole.