810 



TIT,\NOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



gradations) of new cusps on the grinding teeth are 

 developed in the jaw long before birth; obyiously 

 their first appearance is germinal. In apparent but 

 not actual contrast, the rudiments (rectigradations) 

 of horny outgrowths upon the skull first appear in 

 late ontogenetic stages; after a long period of geologic 

 evolution these appear at birth or before birth as 

 germinal biocharacters. This leaves open the ques- 



jBirth of net^ characters, first appearance 

 arly middle or iate stages of deyelopTnen.t 

 '.s rudiments , oro^ressiye and prophetic strucTi 

 Death of old characters, vestiges 

 retrogressive and reversional striutures 



The death plact 



Figure 728.- 



horns first appear as rudiments in adult stages of 

 individual development, but after a very long geologic 

 interval they are thrust forward into prenatal stages, 

 so that the young titanotheres have horns at birth. 

 Such characters are said to be accelerated. Other 

 characters are ontogenetically retarded: they lose their 

 ontogenetic velocity. For example, the lateral digits 

 of the horse are so slowed down in rate of develop- 

 ment that finally they are confined to the 

 germinal stage, in which we find a vestige of 

 one terminal phalanx (Ewart, 1894.1), which 

 is never present in the young (fig. 730, Bi). 

 This gradual acceleration or retardation of 

 biocharacters furnishes one of the most strik- 

 ing examples of continued evolution m adap- 

 tation; one "genus" or "species" slips gradu- 

 ally into another (for example, MerycMppus 

 into Pliohifpus) not through the sudden or 

 abrupt falling out of a character but through 

 the adaptation,, acceleration, or retardation of 

 certain similarly functional biocharacters. 



PJiylogenetic velocity of biocharacters. — Phy- 

 logenetic velocity is a quite different property 

 of biocharacters from that described above. 



-Germinal origin and disappearance of characters 



Showing that the germinal cycle of certain biocharacters, such as rectigradations and certain 

 allometrons, first appears in the germ and that after a long period of acceleration and visible 

 evolution in the soma these biocharacters may undergo retrogressive evolution or retardation 

 and may be retained in the germ as latent or potential predispositions and reappear in the 

 soma only occasionally as reversional structures. This kind of germinal velocity, or character J^ ^g g^ property discovered and demonstrated 



by Osborn in the researches made for this 

 tion whether the initiation of the horns of the titano- 

 there is somatic or germinal. Followers of Lamarck, 

 Spencer, and Cope would contend that as the horns 

 first appear in late stages of somatic development the 

 initiation is somatic (ontogenetic) — that horns first 

 appear as the results of growth, localized by habit. 

 This question is discussed below under the heading 

 "Horns arise as typical rectigradations." 



DIFFERING HEREDITARY VELOCITIES; RATES OF MOTION 

 OF BIOCHARACTERS 



Definitions. — The term velocity applied to a bio- 

 character refers to its separate rate of development 

 in the body (ontogeny) as well as to its separate rate 

 of appearance in evolution (phylogeny). These two 

 kinds of hereditary velocity are quite distiact from 

 each other and are both, in different ways, highly 

 adaptive. The resemblances and contrasts between 

 these two kinds of biocharacter velocity are shown in 

 the following principles and examples. 



Ontogenetic velocity of biocharacters. — The movement 

 of biocharacters has been demonstrated both in em- 

 bryology (Agassiz; Haeckel, biogenetic law) and in 

 paleontology (Hyatt, 1866.1, law of acceleration and 

 retardation). The principle as developed by Hyatt 

 and Cope (1887.1, 1896.1) in paleontology is that 

 biocharacters which may originate at or near adult 

 development of the individual may be inherited in 

 successive generations in earlier and earlier stages of 

 individual development until they exist only in the 

 extremely young or are actually skipped as stages of 

 development. In the titanotheres, for example, the 



