UNIT FIVE — REVIEW • HOW DO LIVING THINGS ORIGINATE? 



Everybody has known for centuries that chickens come from hen's eggs 

 and that great oaks from little acorns grow. But not everybody knows that 

 living things come only from other Uving things more or less like them. And 

 until comparatively recent times, hardly anybody could be sure of this. For 

 there are endless tales of maggots coming out of decaying meat, of horsehairs 

 turning into worms, and of mud becoming converted into eels or frogs. In- 

 deed, many sober-minded persons had reported seeing such things happen 

 under their very eyes. 



Still more difficult has it been to reach clear notions as to just what goes 

 on in the egg to convert it into a chicken; or as to what happens to make the 

 acorn be what it is, with its wonderful capacity to grow at all, or to grow into 

 an oak and nothing else. It has seemed reasonable to ask, Is there a preformed 

 miniature hen inside the egg? Or does formless matter become changed into 

 the organized bird? But we have learned enough to see that the answer is 

 neither one nor the other. There is indeed no miniature hen. But neither 

 is the living part of the egg formless. It is a highly complex and highly special- 

 ized bit of matter that becomes a particular hen, of a particular breed, through 

 an orderly series of changes. And every individual plant and animal passes 

 through an orderly series of changes in much the same way. The transforma- 

 tion of a microscopic germ into an individual involves growth — increase in 

 the amount of protoplasm. And it involves development, a process of becom- 

 ing progressively different. 



We accept the familiar fact that wounds and bruises heal. But we are im- 

 pressed when we see missing organs replaced, even to the extent of making 

 "new individuals" out of fragments of old individuals. These various kinds 

 of happenings, however, are essentially of the same order. We may think of 

 regeneration of common plants and of a few animals as a special aspect of 

 growth: new cells are formed by cell-division. Vegetative propagation or 

 reproduction is an extension of the fact of growth and repair of tissue. More 

 highly specialized is the formation, among most common plants and some 

 animals, of buds or outgrowths which can develop into independent indi- 

 viduals. 



In general, the making of new Individuals Is closely related to the fact 

 that no living beings can continue to live forever. We may think of repro- 

 duction as the continuing of life processes from individual to individual or 

 from generation to generation. Plants and animals almost universally produce 

 special structures or stages that keep "alive" under conditions that do not 

 permit normal metabolism. Spores, seeds, pupae, cysts, protected eggs, 

 survive drought or cold or heat in what is essentially suspended, or extremely 

 reduced, metaboHsm. Whatever goes on inside such structures is more or less 



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