NO. 3 COMPARATIVE HISTOLOGY OF FEMUB FOOTE 25 



there is apparently some undeiiyiiig- determinant which gives to the higlier 

 group a more complete differentiation than is found in a preceding or lower 

 class. That is, the late differentiation in mammals is more complete than it is 

 in birds, in birds than it is in reptiles, and in reptiles than in amphibians; 

 while the early differentiation in each class seems to remain practically the 

 same. As the animal rises in the scale of differentiation, the grade of the adult 

 bone type also rises. As far as the microscopic appearances are concerned it is 

 difficult, if not impossible, to tell when bone units of structure have become 

 complete, but it is not difticult to observe that they have advanced as we go 

 from one class to another. 



2. Geographical position 



The effect of geographical position upon bone variation is not yet reducible 

 to exact deductions. The majority of the femora of amphibians have the first 

 type of bone structure and this, too, regardless of their geographical location. 

 The same is true of the lizards and bats. Perhaps tiie bats furnisli the most 

 important example. As said already, practically the wliole order was examined. 

 The indix'iduals came from all parts of the wiirld where bats abound, and tliey 

 all sliowed the same type of structure with very little variation. Some mammals 

 of different locations are alike in structure and some are unlike. 



In respect to man, the femoi'a of the ancient Egyptians differed from 

 one another greatly, although they were taken from the same cemetery. Tlie 

 same is true of the pre-Columbian Chicama and Pachacamac Indians. In the 

 modern races variations in type are very common and they cannot, in the 

 writer's experience, in any way be associated with geographical position. 



As far as the present observations are concerned, therefore, there is no 

 reason to suppose that geography has had any marked influence upon bone type. 



3. SEX 



In reference to sex, it may be briefly stated that the femora examined 

 shoMed no conclusive evidence that sex was an important factor in the minute 

 structural variation of bone. 



4. age 



Unlike- the previous factors, age influences the type of bone very consider- 

 ably. In the higher mammals and man the femora invariably change in struc- 

 ture with the advancing age of each indi\'idua]. Some femora arrive at com- 

 pletion earlier than others. In the formation of the human bone from early 

 fetal tyi)es there were to be seen distinct evidences of progressive changes from 

 the first through the second to the third type. In some cases this course of 

 development was completed much earlier than it was in others, and senility 



