24 



SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOL. 35 



of 2.5 cm., it was further determined that sections through the middle of the 

 shaft represented the entire structure of the bone with the exception of the 

 extremities. 



Factors Influencing Types of Bone Structure 



1. Grade of the animal in biological classification. 



2. Geographical location. 



3. Sex. 



4. Age. 



5. Function. 



6. Individuality. 



7. Health and disease. 



8. Heredity. 



The results of the investigations in these directions are here briefly sum- 

 marized. 



1. the grade of the animal in biological classification 



It is, perhaps, impossible to decide just how much is evidence in regard 

 to the relation of grade to structure. In the specimens of the different femora 

 examined there were found many variations which, doubtless, have some sig- 

 nificance. On the one hand, there are evidences which tend to show that the 

 grade of an animal has an influence in limiting the structural bone type present; 

 but on the other, there are counter evidences which indicate that the whole 

 matter is not so simple. In support of the first view is the fact that the position 

 which the animal occupies in the scale of life is generally in harmony with 

 the type of bone present in its femur. That is, the lowest class of femoral 

 vertebrates, the lowest order of any class, the lowest genus of any order, and 

 the lowest species of any genus, all show the simplest and most primitive types 

 of bone structure. The converse is also equally true — that the highest class, 

 order, genus, and species shows the most advanced or highly developed type of 

 bone. This may be seen from the specimens, tables, and drawings. While each 

 class, order, genus, and species seems to have a bone cycle of its own, the 

 various cycles are bound together by some factor of an advancing- differentia- 

 tion and the high 1 bone units in one class, order, genus, or species become higher 

 in the next in succession. 



But there are exceptional features which remain to be explained. Each 

 class of animal — amphibian, reptile, bird, mammal — has some first type bone 

 species. Each class, order, genus, and species shows an early and late differen- 

 tiation of bone units. While each class of animal seems to be complete in itself, 



1 The terms high and low do not refer to exact states, but to relative distinctions in differentiation. 



