MECHANICS OF LOCOMOTION 



735 



1. Tibiofemoral ratio = length of tibia -^length of femur. 



2. Radiohumeral ratio = length of radius-^ length of humerus. 



3. Metatarsofemoral ratio = length of Mts III-^ length of 

 femur. 



4. Metacarpohumeral ratio = length of Mtc Ill-f- length of 

 humerus. 



Extreme examples of the adaptive range of these 

 ratios in graviportal and cursorial types expressed as 

 percentages, are the following: 



Tibiofemoral ratio: Lowest (53) in Uintatherium mirahile, 

 a slow, heavy type; highest (125) in Gazella dorcas, a swift, 

 cursorial type. 



Metatarsofemoral ratio: Lowest (10) in Uintatherium mirahile, 

 a graviportal type; highest (103) in Antilocapra, a cursorial 

 type. 



Radiohumeral ratio: Lowest (52) in Pyrotherium, a graviportal 

 type; highest (130) in Neohipparion whitneyi, a cursorial 

 type. 



Metacarpohumeral ratio: Lowest (18) in Mastodon, a gravi- 

 portal type; highest (144) in Gazella dorcas, a cursorial type. 



The widest recorded contrasts in limb ratios are as 

 follows: 



The contrasts between representatives of the chief 

 types of limb adaptation may be further summarized 

 as follows: 



The extremes give us what we may popularly desig- 

 nate speed ratios, weight ratios, mediportal ratios, 

 and primitive ratios. 



Through convergent evolution animals of different 

 ancestry may come to closely resemble one another in 

 limb ratios, as follows: 



Cursorial: 



Perissodactyla, Hipparion 



(Neohipparion) 



Artiodactyla (Antilocapra) _ 

 Graviportal: 



Edentata (Lestodon) 



Amblypoda (Coryphodon).. 

 Brontotheriidae (Brontops 



robustus) 



Amynodontidae (Metamyno- 



don) 



Rhinocerotidae (Teleoceras) . 

 Toxodontia (Toxodon) 



117 

 123 



Meta- 

 tarso- 

 femoral 



101 

 103 



12 

 14 



26 



24 

 25 

 17 



130 

 123 



Meta- 

 carpo- 



116 

 130 



39 

 37 

 38 



We observe that the tibia is relatively long in the 

 primitive ungulates, probably, as observed above, 

 because of their derivation from unguiculate ancestors, 

 in which the tibia is long. The tibia abbreviates in 

 most heavy, slow-moving types; it elongates in all 

 rapidly moving types. 



The radius either remains stationary or abbreviates 

 to a less degree than the tibia in slow-moving types'. 

 In swift-moving types the radius appears to elongate 

 as rapidly as the tibia. 



The metacarpals and metatarsals progressively 

 abbreviate in graviportal types; they progressively 

 elongate in cursorial types. 



RATIOS SHOWING EVOLTTTION FROM PRIMITIVE TO 

 GRAVIPORTAL AND CURSORIAL TYPES 



The contrasts in the limb segment ratios of these four 

 groups having been shown, it is now necessary to point 

 out some of the common characters of primitive 

 ungulates and to demonstrate the fixed laws of evolu- 

 tion into the graviportal and cursorial extremes as 

 well as the limb segment ratios which prevail in the 

 mediportal types. This may be done in the following 

 comparison of a series of groups of types. 



Primitive ungulate types. — The primitive ungulates 

 listed below are partly ambulatory, partly subcursorial, 

 or even cursorial; yet they have in common, as prim- 

 itive or ancestral characters, the elongate tibia and 

 radius and the primitively abbreviate metatarsus and 

 metacarpus — that is, such animals as Euprotogonia 

 and Eoliippus were undoubtedly cursorial, although 

 they lack the elongate metapodials that are charac- 

 teristic of modern cursorial types. 



