844 



TITANOTHERES OF ^^NCIENT WYOMING, DAKOTA, AND NEBRASKA 



some similarly acting influence affecting generation 

 after generation in a similar manner, operating like 

 the "Alutationsrichtung" of Neumayr. Within cer- 

 tain phyla a tendency or predetermination to evolve 

 in breadth or length orthogenetically appears to be 

 established, flowing in one direction like a tide, on the 

 surface of which occur individual fluctuations and 

 variations, like waves and ripples. The overdevelop- 

 ment of certain proportions, as seen in extreme brachy- 

 cephaly and dolichocephaly, for example, seems to be 

 a manifestation of this orthogenetic impulse, which 

 appears to cany certain skulls to inadaptive extremes. 

 As shown in section 2 of this chapter, on the causes 

 of extinction, such extremes of structm-e tend to 

 be eliminated in the struggle for existence and to be 

 replaced by head forms of intermediate development. 



The phenomenon of overevolution has been likened 

 by Loomis to the principle of "momentum" in 

 mechanics. As described below it appears as if 

 certain progressive proportions were continually in- 

 fluenced by certain internal secretions, or organic 

 catalyzers, which continue to accelerate these allo- 

 metric biocharacters in genei'ation after generation 

 and finally to overaccelerate them beyond the point 

 of utility. This is manifest in certain cases of the 

 harmonic, uniform dolichocephaly or brachycephaly 

 of the greater part of the skull, where certain parts 

 are sacrificed or overcrowded, in apparent defiance of 

 the principle of compensation. Again, as macro- 

 cephaly and microcephaly arise as abnormalities due 

 to abnormal internal secretions, it would appear proba- 

 ble that normal internal secretions are in some way 

 connected with the normal evolution of head pro- 

 portions. 



Subject to future experimental test it would appear 

 probable that continuity of head form evolution is 

 not caused through the Darwinian principle of con- 

 tinuous selection of "individual variations" and 

 "individual fluctuations" but by the intermediate 

 influence of some kind of interaction, as implied in the 

 tetrakinetic theory. 



GEEMHTAI All OMETEONS ARISING BY SUDDEIT CHANGES (SALTATIONS) ; 

 INTEEACTION THEORY 



In considering major saltations above we have 

 alluded briefly to the numerous examples of the 

 sudden appearance of germinal allometrons in domes- 

 ticated animals, such as are seen in the short-limbed 

 an con breed of sheep. No evidence has thus far 

 been adduced that any wild breeds of animals have 

 originated suddenly in this way. The fact that all 

 four limbs in quadrupeds are similarly affected by 

 harmonic abbreviation in cases of sudden brachy- 

 mely and that the opposite hands in bipeds are 

 similarly affected in cases of sudden brachydac- 

 tyly points again to the possibility of abnormal 

 chemical messenger interaction as being the imme- 

 diate cause of these kinds of abbreviation. There 



are in man two kinds of brachydactyly — (1) that in 

 which the hand is harmonically abbreviated and 

 (2) that in which the terminal phalanges are lacking. 

 The former, congenital brachydactyly, bears strong 

 superficial resemblance to the ontogenetic brachy- 

 dactyly caused by the abnormal internal secretions 

 of certain parts of the pituitary gland (Gushing, 

 1912.1). This again suggests the hypothesis that 

 congenital brachymely and brachydactyly are due 

 to congenital defects in the hereditary chemical 

 messenger interaction system. The value of this 

 hypothesis can be tested only by physiological 

 experiment. So far as they are understood these 

 phenomena of discontinuity of proportion, like the 

 phenomena of continuity of proportion, appear to be 

 connected with some internal interacting chemical 

 messenger system, which may be at once the seat of 

 balance, of retardation, and of the sudden acceleration 

 manifested in saltations of proportions. 



CERTAIN GERMINAI ALLOMETRONS UNINFLUENCED BY THE DIRECT 

 ACTION OF ENVIRONMENT 



Apart from the well-laiown influence of environ- 

 ment on harmonic size, dwarfing or gigantism, the 

 field, experimental, and anatomical evidence on the 

 influence of environment on germinal allometrons is 

 conflicting. As to field zoologic observations Gerrit 

 S. Miller (letter to author, 1912) observes: 



I can give you plenty of illustrations of "direct action of 

 environment" on color characters in mammals; in fact, you 

 have only to tell me how many you want, and I will fill your 

 order. As to proportions, size, and cranial characters, the 

 case is quite different. So far as I have been able to make out 

 the action is here indirect, as, for instance, when a cat in a 

 wet district takes to a fish diet and [ontogenetically] develops 

 the characters of Aelurus, or when a viverrid does the same and 

 becomes Cynogale. You can hardly say that the differences 

 here observed are due to the direct action of the environment. 

 They are rather due to the ontogenetic modifications caused 

 by habit. I know of plenty of cases of color changes of definite 

 character afl'ecting many different members of a fauna in the 

 same way, as you go from a wet to a dry region, or from a high 

 to a low region, but I do not know of any parallel set of changes 

 in proportions, size, or structure. 



The field zoologist has yet to demonstrate that there 

 is direct relation between certain environments and 

 certain allometrons distinct from the indirect action 

 of environment which may arise as the ontogenic 

 result of differences of locomotion, of food, and of 

 feeding habits in different environments, as, for ex- 

 ample, the influence of the purely carnivorous diet 

 of the Eskimo on their head and jaw conformation. 

 The ichthyologists (Jordan, Evermann) demonstrate 

 that the ontogenic proportions of spines in certain 

 fishes are directly related to the chemical content of 

 their watery medium. 



Anthropologists differ somewhat as to the direct 

 action of environment on the evolution of the form of 

 the human head. Elliot Smith (letter to author, 

 August 12, 1911) finds no evidence that environment 



