448 The American Naturalist. [May, 
the choice and number of the specimens analyzed, but the 
specimens taken for analysis were varied as much as possible 
and in sufficient number as to leave no doubt as to the cor- 
rectness of the final result. There is a progressive increase in 
the quantity of fluorine as compared with the quantity of 
phosphoric acid, between the bones of modern times and those 
of quaternary times; and that the latter contain, on the 
average, six or seven times more than do the bones from the 
tertiary, secondary or primary epochs. 
The tertiary bones contain, on the average, eleven times 
more fluorine than the modern bones, and this augmentation 
appears gradually from one geological period to another. 
The bones belonging to the secondary epoch have a propor- 
tion of fluorine sixteen times more, and those of the primary, 
eighteen times more than the modern bones. 
The bones of the most ancient epochs have almost exactly 
the same proportion of crystallized apatite; the secondary 
bones are not far behind, but the loss becomes sensible in the 
tertiary bones, and more so in the quaternary bones. The 
contrast is still more striking in modern or recent bones, 
where the fluorine is found in a minimum proportion. 
Causes of the increase of Fluorine in Fossil Bones—What can 
be the cause of this progressive enrichment of fossil bones in 
fluorine? How can one explain that this increase has for a 
general limit the proportion of fluorine in apatite, although 
this limit is sometimes exceeded? It seems proper to say “ in- 
crease of proportion of fluorine,” as we could not admit for an 
instant that the bones of ancient animals contained during 
their life the proportions of fluorine which we now find in 
them. Even if this were not opposed to known physiological 
law, it would still become necessary to reject the theory be- 
cause of the considerable difference observed in the composi- 
tion of bones coming from the same species, whether from the 
same or from different epochs. The question arises—what 
could have been the vehicle of the fluorine? We can only 
think of gas or liquid, i. e., of something belonging to atmos- 
phere or water. But as we have no knowledge of any chem- 
ical condition of fluorine under which it could be carried in a 
