CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



847 



biocharacter. In the above example of the evolution 

 of a saltatory (leaping) type of quadruped such as 

 the kangaroo, favorable individual exercise (ontogeny) 

 may carry a hereditary predisposition a little beyond 

 the normal, say from A to B. Thus the saltatory 

 quadruped may be born with a short femur and a 

 Jong tibia, the normal length of which is A. By the 

 habit of long leaping this tibia may be lengthened to 

 JB. In nature this long leaping may have a dele- 

 terious effect on other parts of the aninial or on its 

 progeny, ia which case natural selection would tend 

 to bring the tibia of the jumper back to A. Or long 

 leaping may be beneficial both in the avoidance of 

 •enemies and the procm'ement of food and not dele- 

 terious to offspring, in which case all individuals wall 

 rise to B in innumerable generations until finally 

 natural selection will bring heredity up to B. This 

 illustration affords an example of the relation between 



segments without a corresponding relative increase 

 in the breadth of the skull. Neotoma exhibits a 

 progressive dolichopy and dolichocephaly in ontogeny. 

 The development of the skull of Neotoma exhibits a 

 different ratio in each of the eighteen points measured, 

 just as the evolution of the titanothere skull exhibits 

 a different ratio and index in every bone measured. 

 In passing from adolescence to senility proportions 

 wholly ontogenetic may readily be mistaken for dif- 

 ferences of subspecific or even specific value. In a 

 very large series of crania examined, selected from 

 localities less than 25 miles apart, subject to similar 

 environmental conditions, AUen observed, in addition 

 to the ontogenetic allometrons, a large amount of in- 

 dividual variation both in the form and in the relative 

 size of every element in the adidt skull. He says as 

 to size that there are dwarfs and giants. It is still 

 difficult to determine how far this apparently sponta- 



FiGURE 753. — Femur of dog, normal and as modified b}- amputation or congenital absence of the fore limb 



A, Dog congenitally devoid of fore limbs. A^ Skeleton of the same animal, showing the proportional enlargement of the hind limb and elon- 

 gation of the femur; after Eegnault, 1911. B, Normal femur, tibia, and fibula of dog. C, Femur, tibia, and fibula of a dog whose 

 fore limbs were amputated. B and C after Fuld, 1901. All one-nmth natural size. 



ontogeny and heredity that is established in that form 

 of survival of the fittest that is known as organic or 

 •coincident selection, a theory simultaneously pro- 

 posed by Osborn, Baldwin, and L. Morgan. 



ONTOGENETIC ALIOMETEONS EXPEKIMENTAIIY INFLUENCED BY 

 CHANGES OF ENVIEONMENT 



Certain featm-es of proportion are highly sensitive 

 to envii'onment. The fine, narrow hoofs of the Arab 

 horse, for example, are developed only in reaction 

 to the hard rocky or sandy soil of semidesert regions. 

 If the animal is transferred to the bottom lands or 

 grassy meadows of a humid region its hoofs become 

 broader and flatter. Doubtless all the proportions 

 of the body are subject, either directly or indirectly, 

 to similar ontogenetic modifications brought about by 

 changes of environment. 



GEEMINAl ALIOMETEONS: PRENATAL, ADOLESCENT, ADULT 



Allen (1894.1, p. 234) observes that in Neotoma 

 micropus there is an incessant change in the general 

 proportions of the skull coordinated with growth, 

 due mainly to the lengthening of the several skull 



' neous variability is actually due to environmental, 

 ontogenetic, or germinal mfluence and how far to life- 

 environmental influence. 



A Darwinian interpretation of the evolution of the 

 skull of Neotoma under the hypothesis of organic 

 selection would be that the progressive elongation of 

 the face (dolichopy) and the skuU (dolichocephaly) 

 is attributable to the combined action of natural 

 selection of heritable fluctuations with ontogenetic 

 modifications (allometrons due to feeding habits), on 

 the theory that all germinal variations favoring length 

 of the face and the head are better adapted to the 

 feeding habits of Neotoma and thus would favor 

 survival. Thus congenitally elongate skulls which in 

 course of individual mechanical development most 

 rapidly responded to the process of individual adapta- 

 tion and to the upbending of the face upon the cranium 

 would tend in the long run to be selected. To similar 

 hypothetical coincidence of germinal variation, of 

 selection, and of ontogeny might be theoretically 

 attributed the disharmonic evolution of all the different 

 parts of the skull. 



