846 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



(Ewart, 1909.1). Osbom observed the survival value 

 of these limb proportions among the semiferal mus- 

 tangs of western Louisiana. While the herd was 

 being rounded up one of the mares began to give 

 birth to a colt; she lay down for a moment, the foal 

 was cast, and in an incredibly short time the mother 

 and foal were running with the herd. Stevenson- 

 Hamilton (1912.1, p. 539) observes that within a few 

 hours after birth all young antelopes and zebras, 

 OMTUg to their limb proportions, are able to keep up 

 in the most surprising manner with the full-grown 

 animals, even when those are going at full speed. 



These are instances where the Darwinian theory of 

 the selection of germinal fluctuations of proportion 

 appears to be applicable; whereas the Lamarckian 

 theory of the inheritance of acquired changes of 

 proportion is apparently not applicable. They are 

 notable instances of the high survival value of certain 

 allometrons which are apparently initiated in the 

 germ before they are initiated in ontogeny. 



ONTOGENETIC AllOMETEONS EXPERIMENTALLY INFLUENCED BY 

 CHANGES OF HABIT 



The reverse principle is equally true, namely, that 

 certain adaptive proportions are initiated in ontogeny. 

 Notable instances of this principle are found in the 

 experiments of Regnault (1911.1), whereby it is dem- 

 onstrated that cursorial limb proportions may be 

 transformed into saltatory limb proportions experi- 

 mentally. 



In the normal dog the tibiofemoral proportion — that 

 is, length of the tibia as compared with that of the 

 femur — is 95 per cent. The percentage rises to 104 

 in dogs that are forced congenitally or experimentally 

 into the cursorial habit. Thus a dog born without 

 fore legs sits upright and learns to progress by imper- 

 fect or awkward leaps, in response to which the tibia 

 gradually elongates. Similar results are recorded by 

 Fuld (1901.1) through the amputation of the fore 

 limbs in newborn dogs, in which, through the enforced 

 adoption of the saltatory gait, the tibiofemoral index 



A Bi B 



Figure 752. — Accelerated elongation of the limbs in the young zebra and guanaco 



A, Zebra and colt; B, guanaco; B'. young guanaco. Note the relative length of limb which enables the animal at birth to keep up with its mother. 



Modified after Loomis. 



OTHER GERMINAL ALLOMETRONS OF APPARENTLY NO SURVIVAL 

 SELECTION VALUE; PREDETERMINATION 



No anthropologist has offered any satisfactory 

 explanation as to the adaptive significance of the 

 dolichocephaly and brachycephaly of the human 

 head. Broad heads and long heads have lived in 

 northern Europe for 30,000 years under the same 

 environment, with the same feeding habits and under 

 the same social conditions, and without displaying 

 marked differences of intellectual aptitudes. As to 

 causation Boas writes (letter to author, April 8, 1911): 



So far, the matter [of the origin and cause of head form] is 

 very perple.xing to me. I feel, however, very strongly that 

 changes in type [in head proportion] are very liable to be pro- 

 gressive in definite directions. * * * To my mind it seems 

 no more difficult to assume that this predetermined direction 

 should continue from generation to generation than to make 

 the much more difficult assumption that notwithstanding all 

 internal changes the egg cell of one generation should be 

 absolutely indentical with that of the preceding generation. 



rises to 101-104 as compared with 95 in the normal 

 dog. At the same time the femur and tibia, taking 

 on the entire weight of the body in locomotion, become 

 more massive and exhibit many marked modifications. 

 Contrary to Regnault's opinion (1911.1), this very 

 strildng example of adaptive modification does not 

 demonstrate the Lamarckian principle until it is 

 shown that such modifications are inherited. 



HARMONIC OR CONFLICTING INFLUENCE OF THE FOUR FACTORS OF 

 EVOLUTION 



It is clear from the foregoing and from other 

 examples that will be given that heredity may pre- 

 dispose allometric adaptations which may be de- 

 veloped, intensified, and more or less perfected by 

 ontogeny, by physical environment, and by life 

 environment, or the very reverse. The internal and 

 the external tendencies, if all in the same direction, 

 result in the maximum development of a given 



