Extrinsic Factors.—Agencies outside of the organism itself which in their operation may cause extermination of species may be grouped under the following heads: (1) Unfavorable physical environment; (2) changes affecting the food-supply; (3) preponderance of enemies; (4) the agency of man; (5) cataclysms. A physical environment which can properly be considered unfavorable must act on the individ uals of a species in an adverse manner, and necessarily the result is deleterious to its con tinuance. The, unfavorable characters of the environment may be various, as too high or too low temperatures, excessive moisture or dry ness, unsuitable altitude, storms, winds, pol luted and sediment-laden waters, etc., any of which if not normal to the requirements of the organisms will have a repressive effect on their growth and multiplication. The persistence of these inhibitory conditions will generally cause the speedy extinction of the species. The life of any organism is so dependent upon the sta bility of the food-supply that any change which seriously affects the kind and amount of food reacts immediately upon the species which rely upon it for sustenance. The extinction of a species from any cause involves either the ex tinction of the organisms dependent wholly upon it, or their adaptation to new conditions. Such of these as have lost their plasticity, either f rom high specialization or gerontic develop ment, will perish. The invasion of any prov ince by a new fauna or by a new flora or by both combined will initiate a struggle for su premacy of occupation which will drive out some species, exterminate others and ni.odify still others. In these changes the food is an important element in .determining the succe.ss or failure of a species to maintain itself. On the other hand, abundance of food leads to sluggish habits and tends to produce hicrease size. In this way may be explained the preya lence cig many large contemporaneous species of reptiles in the Mesozoic, and of large mam mals at different epochs in the Tertiary. This increase in size exposed them to increasing danger from starvation, from changes in cli mate and from competition with smaller active and more numerous animals. It is difficult to estimate how far in some cases the extinction of a species may have been due to the invasion of noxious bacteria or other parasitic organ isms. It can only be predicated that their rav ages are sometimes. of such a nature that the ranks of a species are thinned out to the verge of extinction, and it is quite possible that ac tual extermination occasionally has occurred through this cause. Dr. Faltoner believed that insect enemies have prevented the increase and extension of the elephant in India, and Darwin likewise stated that the increase of large quad rupeds in some parts of South America was prevented by insects and blood-sucking bats. It is doubtful, however, whether these agents were ever the primary and sole causes of the extinction of any large animal. The most vul nerable point of attack in the extermination of large animals is their young or their eggs. Ani mals of large size usually lay few eggs or bring forth very few young, and usually at infrequent intervals. The eggs or the young are easily destroyed by small creatures that would be powerless before the full-grown animals. The downfall of the great reptiles at the end of the Mesozoic has been explained by Morris as due (1) to the lack of care reptiles in general take of their eggs or. young; (2) to the smaller num ber of eggs laid by the large species as com pared with the smaller,. whose continued exist ence in some measure is owing to their fecun dity; and (3) to the progressive development of the mammals into egg-eating and predaceous placental forms of higher intelligence, at the close of the Mesozoic.
Agency of Man.:— Man, being the dominant organism of the existing fauna and flora, has since his establishment had an increasing effect in the restriction and extension of contempo rary plants and animals. As compared with the
ordinary forces of nature, which in general work in an almost imperceptible manner when measured by human standards, man's influence has been incisive, profound and very rapid. He has been the only animal that has attempted to conquer nature in an intelligent manner for his own ends. Noxious animals and plants have been persistently attacked until many of them through reduction in munbers or by extinction have ceased to be a menace to his well-being. His inordinate greed has also led him to ex tertninate harmless useful animals by wholesale slaughter, chiefly because they offered little or no resistance, and also from a false notion that nature's resources are inexhaustible. (See Extiricr ANIMALS). Along with man's con scious destruction of life, he has indirectly and without purpose accomplished the extermina tion of species to which he was wholly indiffer ent. This fact necessitates the division of this topic into two parts: (1) organisms directly exterminated by man, and (2) organisms in directly exterminated by man. It is well known that the remains of early prehistoric man are found associated with the bones of extinct am inals. Among these may be mentioned the mam moth, mastodon, cave-bear, cave-hyena, sabre toothed tiger, Irish elk, woolly rhinoceros the giant birds of Madagascar and New Zealand, and many others. While it is impossible to as sert positively that their extermination was due wholly to man, yet undoubtedly man was one of the most powerful agents. Otherwise it is difficult to account for the disappearance of an animal from a continent that in some parts, at least, would furnish the proper climate and food for its continuous existence, from the time of primitive man down to the historic period. Even with the crudest of weapons, man with his superior cunning and intellect could successfully wage a war of extermination on such animals as the mammoth and mastodon by a method already mentioned; namely, that of killing she young.
Coming now to within the historical period, there are quite a number of well-authenticated cases of the extermination of species that can be traced directly to man as either the principal or sole agent. A few instances will be men tioned in this connection, and the list could be easily extended. Probably the best-known example of an animal exterminated by man is the dodo (q.v.), a large flightless bird related to the dove, formerly living on the island of Mauritius. Its clumsy helpless condition made it an easy prey, and the introduction of cats, dogs and pigs into the island hastened its extermination. On the islands of Rodriguez and Bourbon of the same group were found the solitaire (q.v.) and at least two other species of birds related to the dodo. They also soon disappeared after man's occupation of the is lands. The great auk (q.v.) formerly ranged from the northeastern coast of the United States northward to the Arctic regions, and thence along the shores of northern Eu rope. It was at one time extremely abundant, but the last two individuals seen were taken near Iceland in 1844. The great northern sea cow (q.v.) and Pallas cormorant, natives of the Commander Islands, became extinct near the close of the 18th century. The disappear ance of the ure-ox from Europe is also well known. When the Bermudas were first settled in 1612, a food-bird known as the cahow bred in almost incredible numbers on several of the smaller islands. By 1616 it was almost exter minated, and a reference to the bird in 1629 states that it no longer existed. Singularly enough no remains of this bird were discov ered until about 1913 when several cahow skeletons were found in the Bermudas by the naturalist, A. E. Verrill. The sickle-bill, a bird formerly used for making the royal robes in the Sandwich Islands, is now no longer liv ing, having been hunted to extinction.