24 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



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

 shaft represented tlie entire .structure of the bone witli 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 tlie 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 

 lo the relation of grade to structure. In the syjecimens 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, tliere 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 aniuud occupies in the scale of life is generally in harmony with 

 the tyjie of bone ]»resent in its femur. That is, the lowest class of femoral 

 vertebrates, the lowest order of any class, the lowest genus of any order, aii<l 

 tlie 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. Tills 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 lioniid together by some factor of an advanciu'^- differentia- 

 tion and the liigh ' bone units in one class, order, genus, or species become higher 

 in the next in succession. 



But there are exceptional features wliicli renuiiu to be explained. Each 

 class of animal — amphibian, reptile, bird, mammal — has some first ty])e bono 

 species. Kach class, order, neuus. and si>ecies shows an early and late difl'eren- 

 tiatiiiii III' lidiic unit-. While each class of aiiiiiial secnis to be complete in itself, 



' Tlie tonus high and low do not refer to exact states, but to relative distinctions in differentiation 



