808 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Thus four physico-chemical and mechanical factors 

 interoperate with selection according to Osborn's prin- 

 ciple of tetraplasy. Every stage in the development 

 of each titanothere, from youth to old age, was thus 

 tetraplastic ; every visible biocharacter was the re- 

 sultant of these fourfold factors. Natural selection is 

 not in the same sense a factor or a force; it is an 

 arbiter between the individual titanotheres as molded 

 by hereditary predisposition, by life habit, and by the 

 physical and the life environment. These are the 

 observed modes of titanothere development. The 

 hypothetic action of these modes of evolution is con- 

 sidered on pages 835-838 in the discussion of tetrakinesis. 



y/ie !/oi/n^ 

 TTtanotheres 

 of eyery 

 stage 



inherit their 

 btocharacters 

 (rectigradational 

 and proportional) 

 from the 

 germ cells 

 of the parent ^™^ 

 Titanotheres. 

 The body 

 of each 

 Titanothere 



offshoot 

 of the 

 qerm cell 

 of its 

 parent 



Brontot/ierium /eidyi Lower Olk^ocene 



Qenerations o/ Titanotheres 

 between Cocene and Oiiyocene 



Cotitanops qreoorui 



IDDLE LOCENE 



-OCENE 



FiGUKE 727. — Continuity of tlie heredity germ from Eotitanops to 

 Brontops, according to Weismann's theory of the "continuity of the 

 germ plasm" 



PRINCIPLES OF SINGLE CHARACTER (BIOCHARACTER) 

 EVOLUTION 



In the titanotheres, as in other animals, the body 

 is composed of a large number of correlated single ele- 

 ments, characters, or biocharacters, each of which has 

 its more or less independent origin, development, 

 transformation, rise, or decline. These biocharacters, 

 which are separable in individual development and in 

 evolution, are separable also in heredity, a fact known 

 by comparing them with similar biocharacters in the 

 hybridization of living animals like the horse, a 

 perissodactyl remotely related to the titanotheres. 



A biocharacter (Osborn, 1917.460) is any single part 

 or function of an animal which is known to have sepa- 

 rate origin, growth, evolution, ontogenetic or individual 

 rate (velocity) of development, phylogenetic or racial 

 rate (velocity) of evolution, and a presumable separa- 

 bility in the heredity of the germ. A biocharacter 

 partly corresponds to the "unit character" of Men- 

 delisra. 



Examples of biocharacters of the titanotheres are 

 the single cusps of the grinding teeth, the single horns 

 on the skull, the old and new proportions of the skull 

 and of the limb bones. The term is thus very elastic, 

 because it refers to larger and smaller character groups 

 which evolve and are heritable together more 

 or less as units. The term is both bodily 

 (somatic) and germinal in application, for each 

 visible biocharacter may have a number of 

 genes or determiners in the heredity germ. 



Four chief kinds of biocharacters in the teeth 

 and skeleton are common to the titanotheres 

 and other mammals of the paleontologic series, 

 some of which correspond with the heritable 

 variations and fluctuations of Darwin, as 

 shown below. 



1. Saltation biocharacters. — Numerical saltations; sud- 

 denly appearing complete additional parts; multiplica- 

 tion of parts, rarely adaptive (such as supernumerary 

 vertebrae, ribs, digits, teeth), duplication of existing 

 parts (such as duplication of dental cusps, duplication 

 of the tips of the horns) . Similar to certain numerical 

 and "meristic" variations in other animals (such as 

 hyperdactyl}', hyperphalangy). These suddenly appear- 

 ing new characters are complete structures; they are 

 all germinal in origin and are usually abnormal. 



2. Quantitative variations, fluctuations. — Variations 

 (Darwin), chance variations, minor saltations, minor 

 discontinuities (Bateson), minor plus and minus vari- 

 ations; the quantitative and intensive fluctuation of 

 characters around a mean (Quetelet), in a single curve, 

 or in a double curve where two separate evolutionary 

 phyla are hybridized. These minute accidental char- 

 acters include in part the "mutations of De Vries, " 

 which take accidental, chance, or variable directions, 

 as distinguished from the rectigradations of Osborn. 

 All are germinal in origin and are supposed by Darwin 

 to form the chief material out of which selection builds 

 up adaptations through gradual accumulation in one 

 direction. 



3. Rectigradation biocharacters. — Rectigradations of 

 Osborn; new characters developing adaptively, ortho- 



genetically, from the beginning, arising from minute and 

 inconspicuous rudiments (such as new units of structure, 

 hornlets, dental cusplets, new dental folds and plications). 

 These characters are also germinal in origin, but, unlike salta- 

 tions, they develop orthogenetically and very graduall}' and 

 never appear at first as complete structures. They are invari- 

 ably normal. 



4. Proportion biocharacters. — Allometrons of Osborn, quan- 

 titative biocharacters, plus and minus proportions, skull 

 proportions (such as dolichocephaly and brachycephaly) , limb 

 proportions (such as dolichomely and brachymely, adaptive or 

 neutral). These are all proportional, quantitative, in existing 

 units of structure. Unlike saltations and rectigradations they 

 may first appear in ontogeny, in the body, and subsequently in 



