CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHEEES 



809 



the germ. Whatever their cause or origin they become true 

 germinal biocharacters, separable in heredity and developing 

 orthogeneticaUy. 



All these four kinds of biocharacters are observed in 

 many complete series of fossils. In the titanotheres 

 the rise of rectigradation and proportion biocharacters 

 constitutes dominant modes of evolution. The salta- 

 tions seem to be abnormalities or defects in the germ 

 and do not appear to play any part in the modes of 

 evolution of the titanotheres. Theoretically (Darwin) 

 the minor fluctuations of proportion may afford the 

 material out of which certain new adaptive propor- 

 tional characters are evolving. 



The six problems in biocharacter evolution in the 

 body and germ are as follows: 



1. Genesis, initiation of new biocharacters: Do they arise 

 suddenly or continuously? 



2. New proportions: Do proportions arise suddenly or grad- 

 ually? 



3. Correlation: How are biocharacters correlated? 



4. Velocity: How are the motions of biocharacters regulated? 



5. Heritage: What is the behavior of mingled biocharacters? 



6. Genesis: Do new biocharacters first appear in the body or 

 in the potentiality of the germ? 



SEPARABILITY AND COORDINATION OF BIOCHARACTERS 



Although in certain principles of form (morphology) 

 and function (physiology) as well as of inheritance 

 (heredity) biocharacters are separable, in other 

 aspects they are closely coordinated and correlated 

 with other biocharacters so as to play their part in the 

 organism. Consequently biocharacters in course of 

 development display a great number of different and 

 invariably correlated modes of evolution. Five prin- 

 ciples of the evolution of biocharacters are set forth 

 below. 



I. Origin, genesis 



1. Saltations, major. 



2. Saltations, minor; mutations of De Vries. 



3. Continuous genesis of rectigradations. 



4. Sudden genesis of new proportions. 



II. Proportion, genesis 



1. Fluctuation, plus and minus variation. 



2. Continuous evolution of proportion. 



3. Influence of habit. 



4. Influence of internal secretions. 



III. Cooperation, coordination 



1. Compensation in development and degeneration. 



2. Grouping or correlation of function. 



3. Mechanical interrelation of separate parts. 



4. Linkage of biocharacters with male or female sex, respec- 

 tively. 



5. Coordination by internal secretions. 



IV. Motion, velocity 



1. Acceleration in ontogeny. 



2. Retardation in ontogeny. 



3. Acceleration in phylogeny. 



4. Retardation in phylogeny. 



5. Arrest of development in phylogeny. 



6. Acceleration or retardation by internal secretions. 



V. Heritage 



1. Biocharacter heritage, separabiUty, segregation. 



2. Biocharacter heritage blending, intermingling forms of 

 biocharacters from two separate parents or strains. 



3. Biocharacter heritage, "specificity" in strict repetition 

 of parental or racial character. 



4. Separability and blending of proportions. 



INITIATION OF BIOCHARACTERS: IS IT ENVIRONMENTAL, 

 ONTOGENETIC, OR GERMINAL? 



Modes of initiation. — The observed time and place 

 of the visible initiation or genesis of each new bio- 

 character, whether environmental, somatic, or ger- 

 minal, is a matter of first importance. There are 

 contrasts — for example, the visible genesis of each 

 cusp rectigradation is at birth; the visible genesis of 

 each horn rectigradation is in the adult. Does this 

 mean that the new cusps and new horns have different 

 causes? 



In the observation of each biocharacter it is desirable 

 to make the following notes as to its initiation. 



Initiation in origin: Is the first appearance germinal 

 or somatic? For example, all rectigradations in the 

 teeth so far as observed make their first appearance 

 in the germ. 



Initiation in motion (acceleration, retardation, 

 balance, arrest): Is it germinal or somatic? For 

 example, does the initiation in velocity make its first 

 appearance in the germ or in the body? In most 

 animals the acceleration of characters in ontogeny 

 and phylogeny is purely germinal, as in the limb 

 proportions of praecoces (birds and quadrupeds that 

 are capable of rapid motion at birth). 



Initiation in form evolution (rectigradations, allo- 

 metrons): Is it germinal or somatic? For example, 

 although all known rectigradations are congenital, 

 many allometrons may arise first in the soma. 



Initiation in cooperation, coordination, correla- 

 tion, compensation, sex linkage, etc. : Is this germinal 

 or somatic? Besides the congenital correlation of 

 form and function many kinds of coordination arise 

 ontogenetically. 



Germinal initiation. — Close observation of the time 

 of the initiation of each biocharacter, whether obvi- 

 ously environmental in its first appearance or obviously 

 ontogenetic, is very important. Certain proportion 

 characters, such as the proportions of the skuU, ap- 

 parently arise in the germ; other proportion characters 

 may first appear somatically through the use and 

 disuse of parts, as for example in the limbs. We are 

 absolutely siu-e that the initiation of many biochar- 

 acters is germinal, whatever may be their causes. 

 This is particularly true of the rectigradation cusps 

 of the grinding teeth. Again, the rudiments (recti- 



