What is the main purpose of identifying the main idea in a passage?

Directions for Question: Read the passage given below and answer the question with the most appropriate choice.

Passage 4

Every living creature was at some stage of its life nothing more than a single cell. It is generally known that human beings result from the union of an egg cell and a sperm cell, but it is not so universally understood that these germ cells are part of a continuous stream of germplasm which has been in existence ever since the appearance of life on the globe, and which is destined to continue in existence as long as life remains on the globe.

The corollaries of this fact are of great importance. Early investigators tended naturally to look at the germ cells as a product of the body. Being supposedly products of the body, it was natural to think that they would in some measure reproduce the character of the body which created them; and Darwin elaborated an ingenious hypothesis to explain how the various characters could be represented in the germ cell. The idea held by him, in common with most other thinkers of his period, is still held more or less unconsciously by those who have not given particular attention to the subject. Generation is conceived as a direct chain: the body produces the germ-cell which produces another body which in turn produces another germ-cell, and so on.

But a generation ago this idea fell under suspicion. August Weismann, professor of zoölogy at the University of Freiburg, Germany, made himself the champion of the new idea, in about 1885, and developed it so effectively that it is now a part of the creed of nearly every biologist. Weismann caused a general abandonment of the idea that the germ-cell is produced by the body in each generation, and popularized the conception of the germ-cell as a product of a stream of undifferentiated germ-plasm, not only continuous but (potentially at least) immortal. The body does not produce the germ cells, he pointed out; instead, the germ cells produce the body.

The basis of this theory can best be understood by a brief consideration of the reproduction of very simple organisms. "Death is the end of life," is the belief of many other persons than the Lotus Eaters. It is commonly supposed that everything which lives must eventually die. But the study of a one-celled animal, an Infusorian, for example, reveals that when it reaches a certain age it pinches in two, and each half becomes an Infusorian in all appearance identical to the original cell. Has the parent cell then died? It may rather be said to survive, in two parts. Each of these daughter cells will in turn go through the same process of reproduction by simple fission, and the process will be continued in their descendants. The Infusorian can be called potentially immortal, because of this method of reproduction.

The immortality, as Weismann pointed out, is not of the kind attributed by the Greeks to their gods, who could not die because no wound could destroy them. On the contrary, the Infusorian is extremely fragile, and is dying by millions at every instant; but if circumstances are favourable, it can live on; it is not inevitably doomed to die sooner or later, as is Man. "It dies from accident often, from old age never."

Now the single-celled Infusorian is in many respects comparable with the single-celled germ of the higher animals. The analogy has often been carried too far, yet it remains indisputable that the germ cells of men reproduce in the same way—by simple fission—as the Infusorian and other one-celled animals and plants, and that they are organized on much the same plan. Given favourable circumstances, the germ cell should be expected to be equally immortal. Does it ever find these favourable circumstances?

The investigations of microscopists indicate that it does—that evolution has provided it with these favourable circumstances, in the bodies of the higher animals. Let us recall in outline the early history of the fertilized germ cell, the zygote formed by the union of ovum and spermatozoön. These two unite to form a single cell, which is essentially the same, physiologically, as other germ cells. It divides into two similar cells; these each divide; the resulting cells again divide, and so the process continues, until the whole body—a fully developed man,—has been produced by division and re-division of the one zygote.

Question: The author employs Infusorian as an illustration to:

Directions for Question: Read the passage given below and answer the question with the most appropriate choice.

Passage 3

Darwin's work has the property of greatness in that it may be admired from more aspects than one. For some, the perception of the principle of Natural Selection stands out as his most wonderful achievement to which all the rest is subordinate. Others, among whom I would range myself, look up to him rather as the first who plainly distinguished, collected, and comprehensively studied that new class of evidence from which hereafter a true understanding of the process of Evolution may be developed. We each prefer our own standpoint of admiration, but I think that it will be in their wider aspect that his labours will most command the veneration of posterity.

A treatise written to advance knowledge may be read in two moods. The reader may keep his mind passive, willing merely to receive the impression of the writer's thought; or he may read with his attention strained and alert, asking at every instant how the new knowledge can be used in a further advance, watching continually for fresh footholds by which to climb higher still. Of Shelley it has been said that he was a poet for poets: so Darwin was a naturalist for naturalists. It is when his writings are used in the critical and more exacting spirit with which we test the outfit for our own enterprise that we learn their full value and strength. Whether we glance back and compare his performance with the efforts of his predecessors, or look forward along the course that modern research is disclosing, we shall honour most in him not the rounded merit of finite accomplishment, but the creative power by which he inaugurated a line of discovery endless in variety and extension. Let us attempt thus to see his work from a true perspective between the past from which it grew, and the present which is its consequence. Darwin attacked the problem of Evolution by reference to facts of three classes: Variation; Heredity; and Natural Selection. His work was not as the laity suppose, a sudden and unheralded revelation, but the first fruit of a long and hitherto barren controversy. The occurrence of variation from type, and the hereditary transmission of such variation had of course been long familiar to practical men, and inferences as to the possible bearing of those phenomena on the nature of specific difference had been from time to time drawn by naturalists. Maupertuis, for example, wrote: "Ce qui nous reste à examiner, c'est comment d'un seulindividu, il a punaîtretantd'espècessidifférentes." And again: "La Nature contient le fonds de toutescesvariétés: mais le hasardoul'art les mettent en œuvre. C'estainsiqueceuxdontl'industries'applique à satisfaire le goût des curieux, sont, pour ainsi dire, créateursd'espècesnouvelles." Such passages, of which many (though few so emphatic) can be found in eighteenth-century writers, indicate a true perception of the mode of Evolution. The speculations hinted at by Buffon, developed by Erasmus Darwin, and independently proclaimed above all by Lamarck, gave the doctrine of descent a wide renown. The uniformitarian teaching which Lyell deduced from geological observation had gained acceptance. The facts of geographical distribution had been shown to be obviously inconsistent with the Mosaic legend. Prichard, and Lawrence, following the example of Blumenbach, had successfully demonstrated that the races of Man could be regarded as different forms of one species, contrary to the opinion up till then received. These treatises all begin, it is true, with a profound obeisance to the sons of Noah, but when performed, they continue on strictly modern lines. The question of the mutability of species was thus prominently raised.

Question: Except as noted, the author of the passage adopts each of the following positions regarding Darwin: