752 The American Naturalist. [September, 
THE NUMERICAL INTENSITY OF FAUNAS: 
By L. P. Gratacap. 
In the various aspects of the Development of Life upon the 
earth the attention of the student has been principally directed 
to the question of form, as a problem of derivation. The ex- 
ternal configuration of the enclosing frame-work or envelopes 
of organisms, or the modified outlines of internal skeletons 
have been closely compared, and species have been defined 
upon their differences, and the record of the march of specific 
change, group segregation and class development compiled 
from their study. The enumeration of species as they multi- 
ply, or decrease and disappear has been made, and the succes- 
sive expansions and contractions of the lineal avenues of 
descent extensively elaborated. The student has less fre- 
quently been brought to consider the question of number, the 
numerical increase of forms, or to attach any biological sig- 
nificance to the arithmetical rise or decrease of species. It is, 
upon a little reflection evident that the subject of numbers, if 
it admits of any determination, may have or must have, a 
direct connexion with the ease and spontaneity with which a 
new or old species maintains itself, and may prove an index of 
the severity of competition or of the difficulty of living in its 
field of zoological activity. 
Assuming the rate of increase uniform, the apparatus and 
impulse to procreation identical in a number of species, that 
one, of course, will survive in the greatest numbers whose life 
is attended with the least friction, against whose functions and 
habits the smaller array of obstacles active and passive exist. 
The comparison of species in this respect, so far as it is used to 
make out the comparative adaptation of species to certain con- 
ditions, assumes of necessity an identical fecundity in each 
species, and the comparison has, therefore, valid probability 
between species of the same families, or genera or perhaps 
classes. 
‘Paper presented at Brooklyn Meeting of the Amer. Ass. Ad. Sci, Aug., 1894. 
