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

BOOK 4

HOW INDIVIDUALS

Can Life Arise Spontaneously ?

9 I. § 3. Sex is a Complication of Reproduction.

§ 5. Some Evasions and Replacements of Sexuality. § 6. Artificial Propagation. § 7. A Note on the Regeneration of Lost Parts.

CHAPTER 2

ORIGINATE

§ 2. Sexless Reproduction the Original Method.

§ 4. The Gametes, or Marrying Cells.

§ 8. Grafts and the Chimera. § Q.

What is Meant by the Germ-plasm.

§ 1 Can Life Arise Spontaneously ?

Wé have asserted already several times that all life derives from preceding life. But we have given very little to sustain that assertion. Here we will deal rather more fully with this very important issue. Our proposition is that life is a branching tree. We want to dispose of the possibility that it is several branching trees or that new lifetrees have arisen or can originate. We have to answer the question whether under existing conditions living things may not still be able to arise from lifeless material ?

Right down to the middle of the last century it was thought, even by many biologists, that living creatures could so arise—“ spontaneously.” It was believed in not as a miracle, a rare and occasional wonder of nature, but as a fact of everyday experience.

The swarm of busy maggots that appears whenever anything is left to decay was supposed to be directly generated from the putrefying material. In antiquity it was believed that frogs and reptiles could be generated out of mud and slime: Virgil in the Georgics gives a recipe for producing bees from the carcass of an ox: and Samson’s riddle (Judges XIV) seems based on a similar misconception. He saw some beelike flies emerge from the decaying body of a lion and put his riddle: ‘‘ Out of the eater came forth meat, and out of the strong came forth sweetness.”” It was a perfectly natural and reasonable error in the days when the habit of interrogative observation had not yet been developed. But the Bible narrator has gone a little beyond the possibilities of the case by adding a honeycomb to the Story.

It was not until the seventeenth century that Redi exploded the belief that blowflies and their maggots are produced by decaying meat itself. He marked the blowfly at its work. When he prevented flies from having access to the meat by covering it with gauze no maggots were generated. It was a simple, clear-cut experiment, and yet no one had

previously displayed the intelligent scepticism needed to attempt it.

By the seventeenth century the discovery of the microscope had opened up a new world of living creatures, whose life-histories were hard to trace out owing to their complicated life-cycles, to their minuteness, and their liability to be blown from place to place in a condition of nearly-suspended life. These creatures had an air of turning up out of nothingness. Thus they were supposed to be “spontaneously generated.”

In the middle of the nineteenth century the genius of Pasteur, with a combination of rigorous experiment and patient perseverance, finally clinched the matter and proved that all visible living things, at any rate in the conditions which now obtain in nature, arise only from others of the same sort. Soups and such-like infusions were up to his time supposed to be natural environments for spontaneous generation. But the most nutrient of soups will stay pellucid, without a trace of decay, if sealed off from the air after being sufficiently heated to destroy all microbes and their spores. Even if air be later admitted, no decay will set in, provided no bacterial spores actually fall into the soup. They may be filtered off by passing the air through cotton-wool ; or by letting the air pass very slowly along a series of U-shaped bends in a narrow tube, when they will settle at the bottom of the U’s. Or if uninfected air be admitted to sterilized soup, as for instance by breaking open the sealed flask at the top of a high mountain, and the flask then resealed, again there will be no decay. Nothing decays unless bacteria or their spores settle upon it. The same boiled infusion which, left exposed to the air, within a week teems with many kinds of bacteria and protozoa, will remain untenanted indefinitely if their germs and spores and cysts are debarred entrance. We can say now with an entirely reasonable confidence that all life which exists to-day has sprung direct from

re-existing life. o. a But, of Pest, this apparent impossibility of spontaneous generation applies only to the world as we knowit to-day. Atsome time

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