SIGNIFICANCE OF BONE STRUCTURP:. 13 



and that the latter should not. It is very problematical whether the snpra-condyloid 

 foramen of the hnmerns is of advantage to its possessors, and if so, that it would not 

 be equally useful in species that do not possess it. Why is there a supra-condyloid for- 

 amen in the humerus of the seal and none in that of the sea lion? The solid epiphysis 

 of the head of the femur in the kangaroo is not easily accounted for. If it is said that 

 it is to resist the shock of the forcible extension of the femur in jumping, one might ask 

 what explains its appearance, in a less developed form indeed, in the little used antei-ior 

 extremities, or why it is found in the femur of the opossum. To say that this structure 

 is the result of heredity, gives no explanation of its original appearance. If it is useful 

 one would expect to find it in other leaping animals. 



The occurrence of apparently useless rudimentary structures in other systems is so 

 well recognized that thei-e is no a priori reason why they should not occur in bones. 

 Professor Heiberg^ of Christiania has the merit of showing that the lines on the lower 

 end of the human femur, erroneously supposed to be due to the pressui'e of the semi- 

 lunar cartilages, are of this class, being, in fact, the representatives of the more marked 

 separations of the patellar and the two condyloid surfaces in many animals. It is not 

 unlikely that many features of the internal structure of bone may have this significance. 



It is curious that the two most striking peculiarities of the texture of bone that we 

 have noticed in mammals, are those of the seal on one hand, and whale tribes and the 

 manatee on the other. These atjuatic animals resemble one another in the thick layer 

 of fat under the skin and in the flipper-like anterior extremities, though these are rather 

 rudimentary in the whale and highly specialized in the seal, the former having relatively 

 weak muscles and the latter strong ones; but the bones, or at least the larger ones, 

 differ radically in these two types. Each departs from the ordinary plan in an opposite 

 direction, the seal having thick plates and large intei-spaces; the whales and manatee, 

 very numerous thin plates crowded together. It may be urged that the purpose of the 

 heavy cetacean bone is to i-esist pressure at great depths, but though this need may ex- 

 ist for the larger whales, it probably does not for several species, and certainly not for 

 the manatee. The bone being then apparently unnecessarily massive in the whale, the 

 architectural plan of the internal tissue seems doubly unnecessary, yet Ave have seen it 

 beautifully marked in the whale's humerus. In the manatee, especially, there is a strong 

 suggestion of a degenerating type. On the other hand, the structure of the bones of 

 the seal implies at once strength and lightness. 



It appears, therefore, evident that so far from the actual structure of the bone being 

 the only possible one, it in many cases presents useless features, and that certainly there 

 must be some determining factor besides teleology. Is it impossible to hold that the 

 vertebrae of an alligator, for instance, could not answer their purpose equally well if the 

 internal structure were more on the plan of mammalian vertebrae? Are there not pecul- 

 iarities of race that in all cases, at least, do not answer any definite purpose? It seems 

 to me that there can be no reasonable doubt on the matter. It may l)e urged that Ave 

 lack the knoAvledge to decide whether any given organism considered as a Avhole is or is 



' Arcliiv. fiir Aiiat. mid EiilwicUtlung.sseschichtL', 1883. 



