The Meaning of Evolution

by Samuel Christian Schmucker

A little later, while exploring Patagonia, Darwin noticed the terrace-like formation of that desolate country. A flat near the sea was succeeded by a rapid rise, then came another flat. Three of these terraces in succession stretch back toward the Andes. At the base of the high terraces Darwin found marine shells, largely similar to those of the ocean beach so many miles to the east. His study of Lyell led him to suspect at once that this portion of South America had been raised in successive stages out of the bed of the Pacific. When they passed around Cape Horn and up the western coast he hunted for similar beach marks on the sheer western face of the Andes, and found them without difficulty, confirming his idea of the recent rise of this end of the Andean chain.

The Beagle continued its voyage up the western coast of South America until it reached Peru. Once more the abundance of tropical life is under Darwin's eyes, but now it is the life of an entirely different section. The dry climate of Peru furnished him with an environment distinctly unlike that of the moist Brazilian forest. He collects now with avidity, gathering especially insects and birds. Then the ship turned its prow westward across the Pacific, only to stop five hundred miles out at the Galapagos Islands. This little group he studied intensely, collecting large numbers of insects and birds. He had not worked over his collection long before he realized that each island in the group had peculiarities which marked its animals from those of any other island. Whenever two islands were close together in the group the differences in their fauna were found to be comparatively slight. If, however, he examined the animals from two islands lying at opposite ends of the group, the differences were always considerably greater. There was, however, a strong general resemblance among them all and a distant though not so strong resemblance to the corresponding animals of the Peruvian coast. On leaving the Galapagos group, Charles Darwin writes in his diary the suggestive observation that this little group of rocky islands seems to be one of the greatest centers of creative activity. It was this interesting resemblance of the animals of these islands to each other and to those of the Peruvian coast that finally persuaded Darwin that they were all related and were all descended from those of Peru. For the rest of his life, with an intensity which increased with each year, Darwin persisted in a patient search for the possible agencies by which such change could have been brought about. The problem, however, was temporarily eclipsed by a pressing geological question aroused by his visit to the Keeling Atoll. Here his investigation of coral reef formation absolutely captivated him. In the case of most coral islands in the Pacific Ocean the reef exists as a circle of coral enclosing a lagoon of water. In the center of this lagoon stands commonly a rocky island. It is plain that this is the foundation on which the coral built. But, in the case of the Atoll, the coral ring was present and so was the internal lagoon, but there was no rocky island. The key to the solution came with an interesting discovery. Darwin began to put down a grappling iron on the outer side of the reef and to drag up coral. The farther away from the reef he went the deeper was the water from whose bottom he pulled the coral. What at first puzzled him was the fact that so long as he dragged up his coral from depths of a hundred feet or less the coral was alive. Whenever he went to depths of much more than a hundred feet, his coral was always dead, though he was evidently pulling it from situations in which it had grown. Then Darwin remembered the rising Andes, lifting themselves out of the bed of the Pacific. Here was the correlated movement. The bottom of the ocean here was sinking. As it sank it dragged down the corals with it. But the descent was so slow that new corals could build on top of the others fast enough to keep the reef up to the surface of the water. At the rate of growth of coral, this would seem to mean that the bottom could be sinking at a rate of only a few feet a century. But while the reef could keep up to the surface, the rocky island must slowly sink. Darwin inferred that there must be a rocky summit within the lagoon, below the surface of the water. A little sounding soon discovered this island, and the verification of Darwin's theory of coral reef formation was at hand. The description of this Atoll and of his theory of its formation won for Darwin the esteem of geologists when he later presented it in book form.

The voyage was continued around the Cape of Good Hope. Pursuing the usual course of sailing vessels, the Beagle touched once more at Brazil, returning home to England in 1836, after an absence of five years. Charles Darwin himself believed this trip to have been both his education and his opportunity. He had started on it a rather careless and indifferent student. He returned from it the most painstaking and patient naturalist the world has ever known. His father, who had hardly consented to his going because he believed him not stable enough to be intrusted to his own devices for so long a period, was profoundly moved at the sight of him on his return. Believing in phrenology, as did many of the physicians of his time, his father turned to his mother and said, "Look at the shape of his head; it is quite altered"; which, translated into the language of to-day, would read, "How wonderfully the young man has developed."

A part of Charles Darwin's duty to the British Government was to write a narrative of the voyage, and this account of his trip upon the Beagle is one of the great classics of travel in the English language. It won the confidence and respect of a wide circle of readers. In his next book he published his observations made at the Keeling Atoll and announced his theory of the formation of coral islands. This was a distinctly scientific investigation, and it won such immediate favor among geologists as to increase materially the young man's reputation. No one man is ever widely enough acquainted with the animal world to classify all the specimens gathered on such an expedition. In accordance with custom, Darwin began distributing his collections among specialists. Each of these was to identify and describe, to name, if necessary, the kind of material he knew best. Among others, Darwin had a considerable collection of barnacles gathered from boats and wharves in all parts of the world. As he could find no one sufficiently acquainted with these creatures to classify them he decided reluctantly to work them up himself. For about eight years much of his spare time was given to this painfully exacting work. He expresses himself as fearing it was a waste of time. Few systematic workers will agree with him. He did his work so well that it has been unnecessary for anyone to do it again. In addition it gained him the esteem of a new circle of scientists and that a decidedly exclusive circle.

The publication of these books did much for Darwin. His narrative of the voyage gained the good will of cultured England in general. The book on coral reefs won the geologists. His "Manual of the Cirrhipedia" (as the barnacle book was called) secured the attention of systematic zoölogists. The time was not far distant when he would need every aid possible toward gaining and keeping the regard of men; for he was to promulgate a theory that would arouse the bitterest opposition and the keenest scorn.

All the while Darwin was working on these books his mind was quietly busying itself with what he called the species question. The more he studied the material collected on his long tour, the more confident he became that the animals of the present are the altered descendants of the animals of the past. He tried patiently to work out every conceivable hypothesis to see whether he could account for the alteration. He felt quite sure animals changed, but how they changed, and why, he could not for a long time conceive. He knew that gardeners were constantly producing new varieties of plants, and that animals of various breeds were clearly the descendants of other and familiar varieties. Accordingly he began to study the methods of animal and plant breeders, to visit their farms, to open correspondence with them and read all their trade journals, to undertake experiments in the breeding of plants. The longer he worked the more confident he became of the reality of the change; but for a long time no glimmer of the cause by which it could be brought about came to his mind. In 1838 he came across a book by Malthus called "An Essay on Population," in which the author shows that, whereas man increases by a geometric ratio, he cannot hope to increase his food supply in more than an arithmetic ratio. That is, while the food might increase like the series 2–4–6–8–10, the population would increase like the series 2–4–8–16–32. On this basis it is only a question of time when the earth will be too full of people for it to be possible for the food to sustain them. Malthus added many observations and suggestions, but this is as much of the book as interests us in this connection. Here was the idea that suggested to Darwin his agency for producing the change of the animals of the past into those of the present.

The number of animals of any particular species remains practically the same. There may be a few more one year, and a few less another, but on the average, year by year, the number of toads, the number of blacksnakes, the number of field mice, remains sensibly the same. Sometimes the rise of man brings an end to the wild population, and so in the past animals have dropped out of the race. Yet in the long run and for a considerable time the number of any species is constant. But each animal produces offspring in quantities sufficient to far more than replace himself as he dies out. In other words, animals increase not by addition but by multiplication. Too many are born for all of them to live. What becomes of the great mass of them? The answer is they die; most of them die young. Only a few fortunate individuals, favored by being a little stronger, a little more cunning, a little more attractively colored than their mates, survive to carry on the race.

The skillful gardener, looking over his flowers, finds a plant of more than ordinary beauty and thrift of growth. When it comes to maturity he keeps its seeds separate from those of the rest and next year plants them by themselves. As they come up he weeds out all unthrifty plants, only allowing the strongest to come to maturity. As they break into bloom he plucks away all whose flowers do not come up to the high standard he has set for himself. After a while he has but a few plants left, but these are the thriftiest and bear the most beautiful flowers. Again he allows these to mature and selects the seed of the very finest. Next year the process is repeated. After a few generations, usually three if the man is skillful enough, he has a definite strain of flowers that will thereafter come true. This is the process of artificial selection as carried on by man.

Darwin saw that Nature is constantly carrying on a similar process. She produces seeds enough on almost any plant to clothe the world in a few years if all of them could fall into proper ground and thrive like their parents. A friend of mine found a mullein stalk that bore more than seven hundred seed pods and averaged more than nine hundred seeds to the pod, a total of more than six hundred and thirty thousand seeds. If each of these could find lodgment on a plot eighteen inches square, produce a similar number of seeds and plant them all, the result would be overwhelming. The fourth generation would cover land and sea, from pole to pole, one hundred layers deep. But there is no such danger. Year by year the mulleins hold their own and no more. Any particular field may have more or less, but in the long run the average for a district is about the same. Some of the seeds are poor and thin. These scarcely sprout. Others spring up into thin-skinned plants, and the first frost nips them. Still others lack the woolly coating in its finest abundance, and the browsing animals eat these. Others lack power to put out a wide-ranging root supply and the first drought kills these. Still others fail to send up a vigorous stem and the passing animal knocks them over and they die. Of the few that are still surviving, some produce such small and inconspicuous blossoms that the insects scarcely see them, and they go unfertilized. In the end only the aristocrats of the group are left, aristocrats in the best sense of the word. These are strong, thrifty, and beautiful, and are provided with every defense known to the mullein world. From these the mulleins of the next generation will spring. Again Nature will select the best of these, by a repetition of the same process. Thus year by year the stock is improved. Any new feature that is favorable helps its possessor to survive, and, if happily mated, will show itself after a while in the entire group. This, in brief, is the underlying idea of Natural Selection, as Darwin conceived it.

In 1842, at Lyell's suggestion, Darwin wrote a short sketch of his ideas which he, two years later, expanded into a somewhat larger account. The manuscript of these early views of the theory was completely lost and has only been recovered within the last few years. It was recently published under the editorship of Charles Darwin's son, Francis. It is astonishing to see how clearly the first short sketch states the underlying conception which all of Darwin's subsequent work amplifies. Hooker was constantly urging Darwin to write out his whole theory in the form of a book, and Darwin had begun to do so in 1856.