The development of spines, thorns, prickles or horns on animals and plants has been shown by the writer to represent an advanced stage of evolution within the type, as well as the degree of differentiation of the organism, the ratio of its adaptability to the environment and the measure of its vital power. The study of the life histories of spinose forms shows that they are simple and inornate during their young stages, and their phylogeny shows that they were all derived from non-spinous ancestors. It is further believed that spines represent an extreme of superficial differentiation which may become fixed in ontogeny, and also that spinosity repre sents a limit to morphological and physiological variation. Therefore, after attaining the limit of spine differentiation, spinose organisms leave no descendants, and out of spinose types no new types are developed.
The factors as above partly noted affecting the continuance or life of a species may be di vided into two classes : (1) those residing within the individuals of a species itself (intrin sic), and (2) those extraneous to the species (extrinsic). The action of either the intrinsic or extrinsic factors tends to result either in ex tinction or in the mutation of a species into an other form. In both cases a specific type dis appears or is eliminated, although only in the first instance is the species exterminated in the sense that it has left no descendants. Within the limits of this article but little more than a descriptive statement of these, principal factors can be attempted. It should also be borne in mind that there is an overlapping and interde pendency among the factors, causing them to react upon each other. Thus, an unfavorable environment due to change of climate may affect the food (extrinsic), which in turn may affect the vitality of the species, possibly re in degeneracy (intrinsic). Likewise, parasitism and disturbances in symbiotic rela tionships produce far-reaching, complex effects.
Intrinsic Factors of Under this head may be considered such factors as (1) lack of power of adaptation; (2) lack of vitality; (3) overspecialization; (4) old age (gerontic stage of evolution) ; (5) pathologic condition; (6) degeneracy; (7) inbreeding; (8) mutation. When a species cannot accom modate itself to changes of climate, food, etc., or in any way becomes fixed, it must perish un less it can find a suitable and constant environ ment — a physical impossibility. The waning vitality so plainly expressed by many species must be considered as evidence of approaching extinction. Such species usually occupy re stricted geographic provinces, they are generally not numerically abundant, and their powers of reproduction are more or less retarded or re pressed. An animal or plant which becomes so specialized that it can live only under certain peculiar conditions stands a chance of extermi nation whenever the harmony of these condi tions is disturbed. Thus a plant which depends upon a certain species of insect for its fertiliza tion will succumb if the insect seeks other food or is itself extirpated from any cause. Also an animal depending solely upon a species of plant for food, or requiring a certain elevation or range of temperature for its continuance, will be exterminated when these are disturbed, unless it can adapt itself to the change. Spe
cialization in general is manifested by the de parture of organs or sets of organs from what is normal to the class. It results in the extreme differentiation of previous structures, or in their suppression, generally due to disuse or re straint, and also in a perversion of their orig inal function. It has been shown by the study of many instances of extinct species preserved in the rocks of past geological periods, that each species has its period of birth, youth, maturity and old age, which often may be recognized by distinctive individual or numerical characters; so that whenever a species can be shown to possess what are known as gerontic or old-age characters it can be safely predicated that its extinction is near at hand. Pathologic char acters in a species indicate the same conditions as disease in an individual, and point clearly toward extermination. Adverse conditions may affect the entire fauna and flora of a region, producing dwarfed, depauperate and pathologic species. Their history is usually very brief and their places are taken by organisms in accord with the environment. Retrogressive evolution indicates that the race has not only ceased to advance, but is declining. The history of any genetic line of species shows that whenever retrogressive characters appear and constitute dominant features the rapid decline and ex tinction of forms possessing them is imminent The reduction of species numerically, and its restriction within narrow geographic limits, lead to inbreeding and the consequent impair ment of virility. The small herds of European bison preserved by the Russian government in the forests of Lithuania and the Caucasus are rapidly declining, both numerically and in vital ity, due almost wholly, according to recent re ports, to inbreeding. Each species now exist ent must have had an ancestor from which it has been derived through one or more of the many processes of evolution. Some of these ancestral types may be still living, while others are extinct. Going back to past geologic times (for example, to the Carboniferous) each spe cies was derived by evolution from ancestral species. Both the ancestors and all the species once living in the Carboniferous are now ex tinct. Life, however, was continued on into the next age through modified descendants of a percentage of trite Carboniferous species. The rest were exterminated and left no descendants. In the first instance it is extinction by mutation and in the second extinction by extermination. It seems probable that ever since the earth has been fully tenamed with a varied life there has been a fairly constant ratio at all times between the number of species just exterminated, the number of primary species originating by the mutation of ancestral forms, the number of species arising by the special differentiation of the primary species, and the number of species adapting themselves to the changes which are dominant during the succeeding geologic period.