delight in lakes and rivers, and moles are subterranean. As of birds, the aquatic species abound in the polar regions. Marsupials inhabit two widely separated areas -America and Australia. In the latter continent they constitute two thirds of the fauna, while nearly all placental mammals, except bats and a few rats and squirrels, are wanting. Excepting a few species in South Africa and South Asia, edentates are confined to tropical South America. The equine family is indigenous to South and East Africa and Southern Asia, while their fossil remains are abundant in both North and South America. In North America, rodents form about one half the number of mammals; there are very few species in Madagascar. Ruminants are sparingly represented in America. Carnivores flourish in every zone and continent. The prehensile-tailed monkeys are strictly South American; while the anthropoid apes belong to the west coast of Africa, and to Borneo and Sumatra. Both monkeys and apes are most abundant near the equator; in fact, their range is limited by the distribution of palms.

CHAPTER XXV

THE ORIGIN OF ANIMAL SPECIES

THE origin of the immense number of species of plants and animals inhabiting the earth has been a matter of speculation among naturalists and philosophers for many centuries. One theory has held that each species was created separately, while the other, known as the Theory of Evolution, maintains that living forms are derived by natural processes of descent from species that inhabited the earth in earlier times; that is, the ancestral forms became extinct owing to changing conditions of climate, food supply, enemies, and other factors, and their descendants in the course of many generations have become modified in bodily structure and function, these changes leading to the development, or evolution, of the numerous species now living. The evidence in favor of the latter theory is so strong that it is now accepted by scientific men as the true explanation of the mode of origin of all known organisms.

Although the idea of evolution has been more or less definitely held by various naturalists since the time of Aristotle (384-322 B.C.), others, even as recently as Linnæus (1707-1778) and Cuvier (1768–1832), have insisted that species are immutable, or unchanging in characteristics. Bonnet (1720-1793) was the first among later zoologists to suggest that variations of climate, nourishment, and other features of the environment might produce new species, and to use the term evolution in its modern sense; but he adduced no important facts to

support his theory, and it failed to meet with the approval of his contemporaries. Lamarck (1744–1829) afterward adopted this view, collected many facts in its favor, and also advanced the hypothesis, in 1801, that the use and disuse of organs would cause structural modifications in them, producing either increased development or atrophy of parts. These modifications, being inherited by successive generations, would eventually become characteristic of new species thus evolved from the older ones. Lamarck's theory was opposed by Cuvier, the greatest comparative anatomist and paleontologist of his time, who insisted that, if the theory were true, there ought to be among fossils transition forms connecting the extinct with the living species, but that no such forms were known, nor could a process be suggested by which transition could take place. Under Cuvier's leadership the belief became current among geologists that the earth has passed through a series of catastrophes or cataclysms which destroyed all living things, and that it has successively been repeopled with new forms quite unlike those which had perished. The Lamarckian theory passed into obscurity, and was not seriously considered again until it was brought forth for comparison with Darwin's theory of natural selection. The opinions of geologists regarding cataclysms underwent a change after Hutton (1726–1797) urged that in order to understand how the present condition of the earth came about, the changes now taking place must be studied. This view was later vigorously upheld and extended by Lyell (1797-1875), who contended that cataclysms have never occurred, but that the earth has gradually reached its present state through the action of natural forces which are still in operation. Thus the way was prepared for the appearance of the theory which, elaborated and maintained by numerous observa

tions, was propounded by Charles Darwin (1809-1882) in his "Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life," published in 1859. Darwin had served as naturalist on the British exploring ship Beagle on a five years' cruise (1832-1837) around the world, and “was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent." After his return home, twenty additional years were spent in collecting facts, making further observations and experiments, and in pondering the theory before he ventured to publish his results and to state what he regarded as the factors concerned in the process of evolution. A similar conclusion had been reached, simultaneously and independently, by Alfred Russell Wallace (1823- ), who had travelled extensively in South America and the Malay Archipelago, and, like Darwin, had become convinced of the certainty of evolution, and sought for its explanation.

As held by Darwin, the theory of evolution, together with the causes of the process, may be briefly stated as follows:

(1) Organisms tend to produce a great many more offspring than can survive. Linnæus showed that the number of living descendants of an annual plant which produced only two seeds each year would, at the end of twenty years, be over a million. There is, however, no plant known to be so unproductive. With reference to the elephant, regarded as the slowest of breeders, producing at the age of thirty a pair of young, and a pair every thirty years thereafter, and living to be one hundred years old, Darwin computed that at the end of 750 years there would be about nineteen million living elephants all descended from the first pair. Individual insects lay

If all the young

hundreds, and fishes millions, of eggs. were to survive, the earth would soon be unable to supply sufficient food and standing-room.

(2) In spite of this tendency to increase inordinately, the number of animals remains, on the whole, stationary. Even though there may be an enormous temporary increase in the number of certain animals, as in "plagues of grasshoppers," normal conditions are soon restored by natural agencies. Eggs and young are devoured by older animals. Disease, old age, parasites, enemies, storms, floods, cold, heat, drought, and famine are responsible for the death of so many individuals that comparatively few young animals of any species live to maturity.

(3) There results, consequently, severe competition for the necessaries of life, a veritable struggle for existence. In order to thrive, animals need food, shelter from the elements, protection from enemies, and freedom from molestation while rearing their young. Deprivation of any of these is likely to be followed by serious results for the animals as individuals and for the race as a whole. The introduction of sheep has made it impossible for cattle to live on some of the Western ranges, because the sheep crop the grass so closely that there is not enough left to feed the cattle. The "English," or house, sparrow appropriates the best protected nesting places, raises several broods each season, eats whatever food is available, and remains the year round without migrating. By reason of these habits it has been victorious in the contest for the places formerly occupied by native birds. The struggle for existence is most keen between closely related forms, since each will naturally want what the other desires. Until about two centuries ago the black rat was the common rat of Europe. Since then it has been driven out by the brown rat, a larger and stronger species.