242 WHITMAN CROSS 
Mr. Willis speaks of a faunal map as something desirable : 
When the map of formations [lithologic individuals discriminated without 
regard to fossils] is made, the way is prepared for the paleontologist to ascer- 
tain the number and bounds of the faunal units and to draw the faunal 
map." 
While the writer desires to see a discussion of the object and 
practicability of such a map from the paleontological standpoint, 
he desires, with all due diffidence, to make a few comments upon 
this idea. 
A fauna regarded as the aggregate of associated forms living 
at a certain time or during a certain period of the past, embraces 
forms which have run their race and are disappearing, a larger 
number of those which may be said to be enduring, and still 
others appearing for the first time. Any fauna is, then, an 
aggregate of forms whose periods of endurance overlap in most 
complicated manner, not only because they appeared at different 
times, but because some are persistent and others comparatively 
ephemeral. Why should the biologist wish to map the distribu- 
tion of one fauna rather than that of the many others which 
overlap it? If he desires to express conditions of life during 
various epochs, the imperfections of the fossil record, the 
sequence of periods or stages of development, his purpose is 
adequately met by a geological map. 
From another standpoint, there are various contemporaneous 
faunas, and also a flora, the existence of the last being some- 
times overlooked. Thereare marine, fresh water, and land faunas, 
vertebrate and invertebrate, living at the same time but only 
in part represented in any one formation. It would appear that 
the biological needs for maps of distribution could scarcely be 
represented on any one ‘ faunal map.”’ 
It is an old simile to speak of “the book of nature.” Per- 
haps in no line of scientific investigation is the aptness of this 
simile more striking than in the field of geology. The rock 
formations are so many historical documents of varied scope 
t Loc. ctt., p. 560. 
