364 C. A. White—Fossil Faunas and Floras. 
The action of light on photochloride can be a good deal 
affected by placing other substances in contact with it. Any 
substance capable of giving up chlorine seems to influence the 
action somewhat; ferric chloride often acts favorably, also 
stannic and cupric chlorides. 
Evidently an important point in all heliochromic processes is 
that as white light must be represented by white in the image, 
it is an essential condition that white light must exert a bleach- 
ing action on the sensitive substance employed. Red chloride 
does not bleach but darkens in white light, but the property 
of bieaching, to a very considerable extent, may be conferred 
on it by certain other chlorides, and particularly by lead chlo- 
ride and zine chloride. 
This I look upon as very important. 
Another matter of interest is exaltation of sensitiveness, and 
this I find is accomplished in quite a remarkable way by 
sodium salicylate, the presence of which at least trebles the 
the action of light on these substances. And probably on others. 
I am persuaded that in the reactions which have been here 
described, lies the future of heliochromy, and that in some form 
or other this beautiful red chloride is destined to lead eventu- 
ally to the reproduction of natural colors. 
Philadelphia, March 23, 1887. 
Art, XX XVII.— On the Inter-Relation of Contemporaneous Fossil 
Faunas and Floras; by CHARLES A. WHITE. 
IT is apparent to every paleontologist who aims at a compre- 
hensive knowledge of his ‘subject that the idea which has here- 
tofore prevailed, that for every epoch or period of geological 
time certain characteristics have been so impressed upon every 
class of animals and plants which then existed that their re- 
mains, if properly investigated, will be found to bear intrinsic 
evidence as to the particular epoch in which they respectively 
lived, must be accepted with a good degree of qualification. 
We ought naturally to expect to find that the rate of progress 
of evolutional differentiation from epoch to epoch has been vari- 
able, when comparisons are made between marine faunas on 
the one hand and continental faunas and floras on the other, 
because the conditions under which they respectively lived are 
so different; but it is scarcely less apparent that such differences 
have often been very great among the different classes which 
contemporaneous continental faunas and floras have embraced. 
In a former publication for example* I have shown that dino- 
* See this Journal, III, xxvi, pp. 120-123. 
