CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



855 



extinction. A second broad generalization is that 

 unfavorable effects are cumulative — that is, mammals 

 are so closely adjusted to their environment as a 

 whole that one series of unfavorable conditions seems 

 to lead to or to induce another series of unfavorable 

 conditions, and the process of numerical diminution 

 is intensified. 



Life development proceeds under the law of tetra- 

 plasy; consequently we look for the causes of extinc- 

 tion in the four tetraldnetic centers of capturing, 

 storing, realizing, and reproducing vital energy, 

 namely, 



1. Heredity: the germ, germ evolution; reproduction. 



2. Ontogeny: the body, habits and use, modifications, 

 plasticity. 



3. Physical environment: geologic, climatic. 



4. Life environment: plants, animals. 



The older historic classification of these causes is as 

 follows : 

 I. Environmental, external causes: 



1. Geologic and physiographic changes of land masses 



and their connections. 



2. Climatic, periodic changes, secular cold, heat, 



moisture, etc. 



3. Biotic changes in environing plant, insect, bird, and 



mammal life. 

 II. Internal causes: 



4. The law of natural selection. Survival value of 



single characters. Degrees of survival value in 

 systematic divisions. Survival value of minute 

 variations in single characters. Adaptation and 

 inadaptation in size, in certain organs, in intelli- 

 gence, in reproduction, in plasticity or accommo- 

 dation, in immunity to disease, in specialization 

 in the trend of de%'elopment, in the potentiality of 

 further evolution, etc. 



THE lAW OF WATUEAI SELECTION 



In studying the law of natural selection we have to 

 view it in three aspects, as follows: 



1. Selection as it concerns the origin, rise, development, 

 competition, decline, and disappearance of each of the phyla of 

 the titanotheres, for example, and finally of the family as a 

 whole. 



2. Selection as it concerns each of the organs that compose 

 the individual titanotheres, their rise, development, decline, 

 and disappearance. 



3. Selection as it concerns or is supposed to concern each of 

 the "Single characters" (biocharacters) of which these various 

 organs are composed. 



The theory of natural selection was originally 

 stated independently by Darwin and Wallace.*^ 

 Darwin" writes: 



Can it be doubted that * * * ^uy minute variation in 

 structure, habits, or instincts adapting that individual better 



^3 Darwin, Charles, and Wallace, A. E., Onthetendency of species to form varie- 

 ties and on the perpetuation of varieties and species by natural means of selection: 

 Linneau See. (Zoology) Jour. Proc, vol. 3, pp. 45-62, London, August, 1858. 



"^ Darwin, Charles, Extract from an unpublished worlv on species, consisting 

 of a portion of a chapter entitled " On the variation of organic beings in a state of 

 nature; on the natural means of selection; on the comparison of domestic races and 

 true species": Idem, pp. 46-50. Expanded and published as "On the origin of 

 species by means of natural selection, or the preservation of favored races in the 

 struggle for life," 502 pp., London, John Murray, November, 1859. 



to the new conditions would tell upon its vigor and health? 

 In the struggle it would have a better chance of surviving; and 

 those of its offspring which inherited the variation, be it ever so 

 slight, would also have a better chance. 



Wallace^' writes: 



All the variations from the typical form of a species must 

 have some definite effect, however slight, on the habits or 

 capacities of the individuals. * * * if^ on the other hand, 

 any species should produce a variety having slightly increased 

 powers of preserving existence, that variety must inevitably in 

 time acquire a superiority in numbers. * * * Xhe variety 

 would now have replaced the species, of which it would be a 

 more perfectly developed and more highly organized form. 

 It would be in all respects better adapted to secure its safety 

 and to prolong its individual existence and that of the race. 



From 1858 until to-day one link in the Darwin- 

 Wallace chain of thought has been a contested point 

 in the theory of natural selection. It may be para- 

 phrased thus: 



Any minute heritable or germinal variation, be it 

 ever so slight, which better adapts an individual to 

 new conditions will give that individual and those of its 

 offspring which inherit the variation a better chance 

 of survival. Or, stated more briefly and in more 

 modern terms: Germinal variations, be they ever so 

 minute or slight, have a chance of being preserved 

 and accumulated if they have adaptive or survival 

 value. 



As shown in section 1 of this chapter, very careful 

 examination of the actual examples cited by Darwin to 

 illustrate his meaning demonstrates that under 

 "minute variations" he had in mind two kinds of 

 variation, which we now distinguish as follows: 



1. Minute individual variations, heritable, corres- 

 ponding closely with the "mutations of De Vries.' 

 These Darwin believed to be the chief cause of 

 evolution. 



2. Variations of proportion, now known as fluctua- 

 tions, as in Darwin's explanation of the origin of the 

 long neck of the giraffe, where he observes that any 

 increase in the length of the neck of a giraffe would 

 increase the animal's chance of survival in times of 

 drought. The evidence for these hypotheses will now 

 be reexamined, and the conclusions regarding natural 

 selection will be summed up at the end of this chapter. 

 These hypotheses were also examined at the end of 

 section 1, especially in relation to the principle of 

 tetraplasy and the tetrakinetic theory. 



Does the actual order of evolution of the titanotheres 

 favor the original Darwin-Wallace hypothesis? Do 

 these animals evolve in the Darwin-Wallace way? 



The independent development of hundreds if not 

 thousands of distinct characters in different parts of 

 the individual titanotheres has put before us with 

 crystalline clearness this crucial test of the Darwin- 

 Wallaxje theory of selection — namely, what observable 



" Wallace, A. E., On the tendency of varieties to depart indefinitely from the 

 original type: Idem, pp. 53-62, reprinted with slight changes in " Contributions to 

 the theory of natural selection," London, Macmillan & Co., 1870. 



