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

HOW INDIVIDUALS ORIGINATE

these gemmules, as they are called, fall to the bottom ; when conditions are favourable again, they burst their case and develop into new sponges. The interesting fact about these gemmules is that they do not originate at a single centre of development, but are built out of scattered cells which come together from different parts of the body.

Plants, from the lowest to the highest, possess the most varied methods of sexless reproduction. Ferns and fungi, mosses and seaweeds, detach myriads of single cells or spores, each capable of growing directly into a new plant. The thread-alge, and most fungi through the major part of their lifehistory, consist of a feltwork of filaments, any bit of which may become detached and continue the race. So with many flowering plants which have creeping underground stems: they send up shoots at intervals, and any one of these, if detached from the rest, can lay claim to be a new-reproduced plant. The strawberry grows new plants at the end of its runners ; poplars and many other trees send out suckers from their roots, each of which can grow into a whole new tree. The banyan tree lets fall aerial roots from its branches, each of which on touching the ground takes root and becomes a new trunk, so that a many-trunked tree, a grove in itself, is produced.

§ 3

Sex is a Complication of Reproduction

A consideration of these strange, asexual methods of reproduction leads to some interesting and important generalizations.

It is evident in the first place that reproduction is essentially nothing more than a special kind of growth. It is growth accompanied by detachment. A bit of the parentbody is split off ; a piece of living substance, instead of growing on as a part, grows into a new whole. Reproduction is not, in any strict sense of the word, creation. Nothing is suddenly called into being. It is simply a separation and a remodelling of part of the parent organism.

We come now to a difficult and fascinating problem, the problem of sex. What has sex to do with this reproductive process ? Why, if creatures can multiply by mere division, should the complication of sex intrude into the life-cycle ?

If we trace very briefly its evolution, and show how it presents itself among the simpler forms of life, we shall discover a fact which to

us vertebrates seems startling—that essentially sex is not reproductive. Jt is a different thing from reproduction.

In the bacteria there is no sex. There is simply sexless proliferation. The creatures divide and divide by binary fission, by tearing themselves into halves, and as far as we can see they get on perfectly well without any form of sexual union. Thus the simplest living things to-day ; and thus, presumably, life began.

Among the microscopic — single-celled animals and plants we see the beginnings of sexual union. We see it appearing as a new intrusive process, perfectly distinct from reproductive proliferation, interrupting and delaying the latter and in its essence antagonistic to it. In the simplest flagellates, for example, the organisms multiply by binary fission just as bacteria do, but their lifehistory is complicated by a contrary tendency. Occasionally, if we are watching the creatures through a microscope, two individuals may be seen to come together and to melt completely into one. It is, of course, a much rarer event than normal fission—otherwise the species would not increase—and the individuals taking part in conjugation, as this union is called, often come from different and not very closely related stocks. And it is as obviously unrelated to reproduction as is feeding or excretion (Fig. 159).

In other kinds of protozoa the process is varied in divers ways. In the highly organized Ciliates, for example, two individuals come to lie side by side and then exchange bits of their nuclei. Here the fusion does not involve the whole organism, but, nevertheless, there is a definite mixing of material from different strains. Here also the process is anti-reproductive, because it occupies time which might be spent in the normal rhythm of growth and fission; actually the act of union takes about as long as would three generations of ordinary fission.

In the simpler many-celled animals and plants—the slime-fungi, for example, or the seaweeds—the gradual entanglement of sex with reproduction can be seen. They reproduce by liberating clusters of tiny dancing flagellated cells whose business is to grow into new individuals. But before they do so these cells generally come together in pairs and melt into one. Here then the . business of sexual conjugation 1s relegated to the reproductive cells. Their reproductive value is evidently diminished by this process which halves their number; but it has another compensating purpose, for the

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