Zoology Books PDF free download 2020

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Introduction

EVOLUTION SEEN IN PERSPECTIVE

Organic Evolution

Evolution means change. We speak of the evolution of the solar system, the evolution of the earth, the evolution of the airplane, the evolution of the automobile. In such cases, we are referring to the changes which have occurred in the solar system, earth, airplane, or automobile.

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The evolution with which this book is concerned involves a special form of the broader meaning of the term: organic evolution. This subdivision of evolution deals with changes undergone by living things, plants, and animals.

For our purposes we may define organic evolution as the theory that plants and animals now living are the modified descendants of somewhat different plants and animals that lived in times past. These ancestors, in their turn, are thought of as being the descendants of predecessors which differed from them, and so on, step by step, back to a beginning shrouded in mystery.

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In the above statement the words “modified descendants” deserve special emphasis. The word “modified” refers to the element of change which we have just mentioned as inherent in the whole idea of evolution.

The word “descendants” introduces an idea not present in the broader use of the term “evolution.” When we speak of the evolution of the automobile we have reference to the changes occurring in the transition from the “horseless carriage” of a bygone era to the model currently advertised.

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We do not think of the older automobiles as being the parents or ancestors of the newer ones in any literal sense. Makers of automobiles learn from experience gained with older models how to improve and modify their product so that later models are different from, and on the whole better than, earlier ones. But the later models are not literally the offspring of the earlier ones.

Contrariwise, it is exactly this ancestor-descendant relationship which is visualized in the term ”organic evolution.” More recent animals are thought of as the direct genetic descendants of somewhat differ- ing ancestors which formerly lived on the earth. The reader will have noted that the definition of organic evolution just given differs from the popular conceptions of the meaning of evolution.

If the proverbial “man in the street” is asked the meaning of the word, he is likely to reply, “Man came from monkeys.” This exclusive preoccupation with man is perhaps natural in one but little acquainted with,

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or interested in, the remainder of the living world. To a biologist, the evolution of man is but one portion of the vast drama of evolutionary change including all living things. Each animal alive today is the product of long evolutionary history.

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