The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

needs. Extend the parallel one degree further ; contrast the day-to-day routine of that one-man community the savage, with the enormously complicated and varied activities of a whole civilized nation, and you get a valid parallel to the difference between ameceba and your own highly organized body.

§ 2 The Minutest Animals

Ameeba belongs to an exceedingly large and varied phylum of minute animals, the Protozoa. It is the simplest, the least organized member of the group. Most of the protozoa have definite and enduring shapes, and special organs such as lashes or vibrating hairs with which to propel themselves ; usually they take in food at some special mouth-point on their surface. But amceba presents in its nakedness the essential features of protozoan organization; for nearly all members of the group are invisibly small so far as the naked eye is concerned, and their structure is comparable to the structure of a single cell of the higher animals.

The protozoa, having for the most part unprotected surfaces over which their chemical interchanges take place, are confined to wet or damp situations ; they cannot exist in the absence of moisture (except as encysted or resting forms). But nearly everywhere where moisture collects they swarm, and they may produce the most far-reaching effects.

The reason why such minute organisms can be so very important, both in the general economy of Nature and to ourselves, is at first sight a paradoxical one. It is because their lives are so short. To show how this may be, let us return to amceba. We have seen how it is built, how it eats and grows ; let us see how it increases and multiplies.

The simplest method—for there are several —is, by our standards at least, a very curious one. It is like the multiplication of cells in one of the higher animals. When the creature has lived for a time and grown to a sufficient size it simply pulls itself into two halves, each part getting half the nucleus, and the resulting fragments creep away and live free and independent lives of their own. ‘They wander about and nourish themselves and grow, and in course of time, when they are big enough, they divide in their turn. And soon. ‘This simple splitting, this tearing of oneself into two halves is the chief method in which the protozoa multiply ; it is, so to speak, their routine method of reproduction. ‘They have other ways. ‘They may vary the monotony of continual splitting by a primi-

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CHAPTER 6

tive kind of sexual union, or sometimes instead of dividing into halves they may chop themselves up into a greater number of bits, into a sort of living mince each particle of which can grow into a new individual. But that need not concern us here; for the present we will confine ourselves to the commonest method of splitting into halves.

The time that intervenes between successive divisions varies in the different kinds of protozoa. In the larger species an individual may live for days or months before he feels the call to tear his body into living bits. But in the smallest species the time is shorter; it may be as brief as an hour. Let us consider one of these shortlived kinds; let us start with a single individual and imagine that it has no enemies and all the food it wants. It will flourish and grow, and at the end of an hour there will be two. At the end of two hours there will be four. At the end of three, eight. And so on, the number increasing ever more rapidly. At the end of thirty-six hours there will be two raised to the thirty-sixth power, which is sixty-eight thousand five hundred million—over thirty times the number of people in the world. This at the end of a day and a half! Imagine that the creature was one of the smallest free-living protozoa, one of the monads that can only be clearly seen with the highest magnification of the microscope, this thirty-six hours’ proliferation would be enough to form quite a respectable heap on a shilling.

Let the experiment continue. In another thirty-six hours the number will be squared —there would be enough to make heaps on seventy thousand million shillings. In a week there would be a number so inconceivably vast that it would be waste of space to write it down, and the monads would together weigh many times more than the whole earth.

This is, of course, an impossible experiment. It is true that most protozoa are short-lived, and that they tend to multiply at this rate, but under natural conditions their numbers are kept down by factors outside themselves, by limitation of the foodsupply, by hungry enemies. Nevertheless, the calculation shows the almost explosive expansion of a protozoan population under favourable conditions. And in this fact lies the power of these microscopic creatures. The countless swarms of micro-organisms in the sea are eaten by minute crustaceans, and these in their turn are eaten by fish and the fish by other fish or by ourselves ; constantly and by the million they are being