on the Conformation of Bones 377 



functional value is not clear. Certainly in the rabbit it may experi- 

 mentally be deprived of its femoral attachment without noticeable 

 difference in the animars locomotion; though it is possible that 

 minute differences would be found in its skill at turning rapidly 

 or in the placing of its feet when at full speed. 



A hyrax {H. cccpensis), which Prof. Keith kindly placed at my 

 disposal, presented an interesting intermediate stage, foreshadow- 

 ing the "long vastus." The superficial gluteus was attached not 

 only to the femur, but its anterior fibres were adherent to the 

 femorococcygeus, which passed down to the patella as in the cur- 

 sorials. 



Most jumping animals present very similar changes in the 

 femorococcygeus. Whatever the group of origin, whether a cat, a 

 cow, or a kangaroo, the hne of speciahsation is similar. 



In the Primates the femorococcygeus tends to disappear or 

 become incorporated in the superficial gluteus and to lose its lower 

 femoral attachment by regression or migration of its lower fibres. 

 The superficial gluteus (i.e. gluteus maximus) is attached just below 

 the great trochanter to a more or less rudimentary third trochanter. 

 The lemurs, however, and the giant apes retain the femorococcy- 

 geus attachment along the femoral shaft; they are not speciahsed, 

 as are the monkeys, for a modified arboreal hfe, and man for a new 

 mode of progression. That the gibbon presents in this respect to 

 some extent a parallel evolution with monkeys and man is sup- 

 ported by the fossil Paidopithex whose femoral shaft is more like 

 that seen in lemurs and the gorilla {vide infra). 



In man traces of the femorococcygeus are still to be found in 

 the external intermuscular septum as low down as the condyle — 

 just in the position which the muscle occupies in the gorilla. The 

 primitive condition found in the gorilla, chimpanzee, and orang, 

 is probably truly primitive. There is no reason to suppose that this 

 condition is ever acquired secondarily after the speciahsed features 

 found in cursorials, jumpers, and monkeys have arisen. There is in 

 fact reason to beheve that Irreversibility of evolution of a muscle 

 Hke the femorococcygeus is as true as that of many other organs, 

 such as teeth. Various kinds of mammahan speciahsation alter 

 its primitive arrangement; to this it will not revert, but would 

 more probably merely atrophy, with lethargic habits. 



To prove the condition in the gorilla to be secondary to that in 

 catarrhine monkeys requires proof that the "primitive" femoro- 

 coccygeus is more suited to him than, say, the catarrhine arrange- 

 ment with a biceps which has extended its fascial attachment up- 

 wards to the middle of the thigh, and with complete absence (as an 

 independent muscle) of the femorococcygeus. 



The plasticity of muscle and of bone is of the same order; they 

 both show phylogenetic as well as ontogenetic response to function. 



