4 SMITHSONIAX MISCELLANEOUS COLLECTIONS VOL. 6l 



development of the subject. Some femora, however, arrive at a com- 

 pletion, or rather at a cessation of the changes, regardless of the stage 

 reached, earlier than others. The only future change in such bones 

 is senility. The progress, however irregular or incomplete it may be, 

 is always from the first towards the second or the third type, never the 



reverse. 



5. The effect of function upon variation in bone structure can 

 scarcely be doubted, but the exact causes and effects are as yet difficult 

 to determine. In the study of 50 genera of bats, the small Pteropus 

 presents still, like the rest of the bats, the first type of bone structure, 

 while the large Pteropus shows already an early and crude third 

 type in process of formation. In a turkey of 16 pounds weight only 

 the second type appeared, while in a turkey of 32 pounds weight 

 there was noticeable a number of Haversian systems. In many 

 femora of all classes the linea aspera, the most " functional " part of 

 the bone, is composed chiefly of third type units regardless of the 

 type of the rest of the bone. Finally, a lack of function in an adult 

 bone doubtless favors an earlier setting in of senile changes. 



6. Individual variations are rare in the lower vertebrates and 

 increase in frequency in the higher forms. But they are mostly of 

 secondary importance, the characteristic structure in species remain- 

 in^ pretty true. The slight variations present are probably partly 

 accidental, partly hereditary and partly functional. 



7. Variations due to health and disease remain very largely for 

 studies in the future. They will be almost wholly restricted to man. 



8. Hereditary influence finally, is clearly demonstrated by the pre- 

 dominance of a certain form of structure in every given species. In 

 families the subject needs much further attention. 



DETAILS CONCERNING GENERA 

 AMPHIBIANS 

 The amphibians present the following conditions : 



1. Simple first type bone with round and oval lacunae and few 

 canaliculi. In some forms cancellous bone occupies the medullary 

 canal ; this is seen, for instance, in the Amblystoma tigrinum, one of 

 the most primitive amphibians. 



2. A division of the simple bone into two concentric laminae, 

 external and internal. 



3. A differentiation into external, central and internal laminae ; and 



4. In a few amphibians, as in the Toad group, very crude Haver- 

 sian systems become outlined in the central lamina. These primitive 



