EXTINCTION OF SPECIES. The an tithesis of evolution is extinction. Direct evo lution leads by insensible degrees to replace ment by changed descendants or to actual de cadence and disappearance of types. Study of the phenomena of elimination sheds much light on the nature of life and the general evolution ary course. Also, the fossil record peculiarly lends itself to such a study, being so largely a record of extinctions.
The particular reasons for the extinction of animals and plants are not always obvious and may be very difficult of interpretation. Barring some factors to be mentioned later, the same variations of environment and most of the forces of selection considered as necessary to the production of new species are also powerful agents in the extermination of established forms. The history of many species may be compared to that of an individual. It has its birth, its growth, its decline, its death. Also, the laws of evolution show that many other species undergo modifications or changes which ultimately transgress or supplant the original assemblage of specific characters, so that a new species results by a process of mutation. Fur thermore, many other species, while retaining their strictly specific characters, may be trans lated into different genera through modifica tions of their generic characters.
The continuance of a species depends upon its harmony with its environment. A perfectly stable and continuous environment is obviously a natural impossibility. The physical conditions of any region of the earth are in a state of con stant change, sometimes very gradual and ex tending over long periods of time, sometimes sufficiently rapid to be measured by ordinary standards. The organic agencies surrounding any species are also not permanent; migrations are continually going on; the areas occupied by various organisms are being extended or re duced; periods of excessive or repressed fecun dity often occur ; there are times of abundance and scarcity of food, increase and diminution in the number of enemies, and so on. Any ma terial change in the physical or organic environ ment must produce a readjustment among the individuals composing a species; their number may be lessened or increased, or they may be forced into conditions of life which produce changes in habits, place or abode, food, func tion, structure or organs.
The study of a geographic life-province shows that the organisms inhabiting it are in the state of a moving equilibrium. Minor changes in the physical conditions, as slight differences in temperature, moisture, elevations, etc., may be compensated for by a readjustment among the organisms themselves. In some cases this readjustment may be favorable to many of the species, while in others it may initiate changes which ultimately result in ex tinction. More profound changes in the physical environment necessarily produce a greater effect upon the animals and plants, and may result in the extermination of many and the considerable modification of others, so that a distinctly transmuted fauna and flora would occupy the The forces already mentioned, though opera tive to a greater or less degree, are not believed to affect so immediately the equilibrium of a species or the general equilibrium of a biotic province as the invasion of new species, either by extension, migration or evolution. The struggle for existence amid gradually changing physical conditions alone is not so aggressive as the invasion of a new assemblage of plants and animals; for in the former the struggle is the normal result of the physical and organic forces of an environment in which the adjust ments have been made and an equilibrium reached; in the other there is the direct addi tion of a new set of oppoting forces, requiring the immediate readjustment of both invading and invaded organisms.
A census of the animals and plants of any region will show a great variation in the num ber of individuals representing the various spe cies. Some are abundant, sonic common, while others are rare. Now, since the normal im pulse of the individuals of each species is to increase inordinately, the fact that some are of rare occurrence shows that forces are at work tending to check their multiplication, and tke relative rarity of a species, as compared with others of the same genus, is taken as an indica tion of approaching extinction.