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

BOOK 2

CHAPTER 6

THE

LOWLY AND

The Minutest Animals.

S 1. Ameba. § 2. § 4. Moulds, Toadstools, and Yeasts. § 7. Bacteria.

§ I Ameba

AG yet we have found little employment for the microscope in our survey of the forms of life. Here and there we have met creatures too small to be easily examined, tiny worms and crustaceans and mites and rotifers, but for the most part they were miniatures of the larger animals that we can see around us ; we have used the microscope only for prying into the finer details of the anatomy of larger living things. But we must plunge now into a world of creatures which are nearly all minute, attaining only in exceptional instances such dimensions as to be seen with the naked eye.

It is a strange and exciting world. Its existence was hardly suspected until the seventeenth century, when Anthony van Leeuvenhoek a Dutch draper, peered through lenses of his own making at drops of water and bits of cheese and the scrapings of his own teeth—indeed, at anything he could lay his hands on—and beheld for the first time their curious inhabitants.

An enormous and vitally important branch of biological science has sprung from these observations. Nowadays everybody has heard of ‘‘ microbes,” tiny insidious creatures, populating the earth with unseen multitudes. Because of their minuteness the smallest drop of water is a habitable world for them, and a few particles of decaying organic matter are enough to support a thriving microbe population. They swarm in pools and puddles, in the moisture of the soil, in the moisture that clings to mosses and other plants. Their spores float everywhere in the air. Ifa few blades of straw or a little pepper or some other organic substance be added to water, and if it be left in a glass, it will be found in a few days to be teeming with these tiny creatures. Moreover, their species may attain to inconceivably vast numbers. The number of men in the world to-day is about seventeen hundred millions, and the numbers of individuals in most species of the larger mammals are probably lower than this. But microbes attain numbers almost beyond

°

MINUTE

§ 3. Plant-animals and Seaweeds. 8 5. Lichens. § 6. Slime-moulds.

§ 8. The Smallest Living Things.

estimation, numbers that make such figures as these negligibly small. The green colour of stagnant ponds is due to microscopic organisms, individually too small to be visible ; the colour of the Red Sea is due to untold myriads of microscopic plants, ‘‘ making the green one red.’ ‘There are microbes which invade our bodies and rack us with a variety of diseases, there are others which are not enemies but indispensable friends. And aside from its practical importance there are beauty and wonder in the micro-world. Few poems have been written about the green soupiness of stagnant ponds or about the green on the boles of trees or about the powdery granules of flinty earths ; nevertheless, as we explore we shall find creatures beautiful and fascinating and of the profoundest interest to anyone of a philosophical turn of mind.

Abundant in the greenish, apparently lifeless scum that accumulates at the bottom of most ponds, water-butts and the like is the curious, elementary little creature known as Ameeba. It is a speck of a transparent jelly-like substance, about one-hundredth of an inch long, showing very little organization in its frankly visible inside. The particular individual in Fig. 113 is seen from above, crawling—or rather flowing—over a flat surface ; from the bird’s-eye point of view of the artist it is proceeding expansively towards the left. It is a creature vastly more simple than any that we have hitherto considered. It has no tissues, no definite organs; no heart, no brain, no stomach, no kidneys, no bones. Except for two special structures that will be considered in a moment, 1t Is nothing but a spot of jelly without enduring form—but a jelly that lives—breathing by absorbing traces of oxygen and giving out traces of carbon dioxide, and moving about and feeding itself in a peculiar and characteristic way. .

In this jelly two layers may be discerned, a comparatively opaque and very granular inner layer and a perfectly transparent outer layer, more solid in consistency than the inner. This layering is not permanent ; the substances of the two layers mingle and

167