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

BOOK 4

in the remote past, when the earth was hotter and its air and crust differed, physically and chemically, from their present state, it seems reasonable to believe that life must have originated in a simple form from lifeless matter. It was presumably a fairly gradual change, a slow progressive synthesis, rather than a sudden leaping into being of organisms from formless slime. To that problem we shall return. Our present conclusion is that, although life could be generated under those strange conditions, it never appears

Fig. 156. One of Pasteur’s experiments.

When yeast infusion is boiled in a swan-necked flask, no putrefaction follows, even though it is left open to the air, because any living germs in the air settle in the bend of the U or are caught on the moist sides of the neck. If the neck is broken off, putrefaction sets in, since spores and

cysts now have ready admission.

spontaneously at the present time. The conditions have disappeared. The animals and plants that we know to-day are branches of a single Tree of Life, growing out of preexisting branches by the reproductive processes that we have now to examine.

§ 2

s

Sexless Reproduction the Original Method

Among most of the higher animals reproduction, the origination of new individuals, is inseparable from sex. It takes two

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THE SCIENCE OF LIFE

CHAPTER 2

to make any addition to the race. But that is by no means true of life generally. Nearly all living things have some kind of sexual process, but most of them have also sexless ways of increasing their kind. In biological language, reproduction is often asexual. Our own strict adherence to a single generative technique is in fact to be regarded as the exceptional thing.

The most primitive way of multiplying, the method used by the smallest and simplest creatures, is by splitting the whole body into two halves, each of which grows into a complete new individual. This is known as binary fission (Figs. 114, 157). In such cases the offspring is not merely a detached part of the parent ; the whole substance of the parent becomes offspring. The parent leaves no corpse. It ceases to exist ; but for it there is no death—only duplication.

This reproduction by splitting, this multiplication of substance passed on into ever new individuals, is most spectacular in the bacteria. A simple bacterium in a congenial nutrient soup may easily accomplish its whole span of individual existence in half an hour. At zero hour, we have one specimen: in half an hour, two specimens—five hours, a thousand and twenty-four —ten hours, over a million—twentyfour hours (if the food holds out) hundreds of billions.

When the dividing animal has a complex structure, remarkable rearrangements must occur at each act of fission. Thus the complicated protozoan Stylonychia (Fig. 157), although it is only a single cell, has a definite shape, with special bristles which act like legs, and a gullet armed with vibratile plates. During fission, many of the old organs disappear, new organs appear in each future individual, and, while division is being effected, develop into their definitive forms and migrate to their definitive stations.

Binary fission, we may note, is by no means unknown in many-celled animals. Not only do many sea-anemones reproduce by splitting themselves slowly through from the mouth downwards, but this longitudinal fission is one of the main methods by which the huge colonies of corals are built up from asingle original polyp. ‘Transverse division, on the other hand, occurs in various worms, especially Planarians.