58 
ISLAND LIFE. 
[part. I. 
question of differences of age or sex, and the pair last referred to 
were taken at the same place and on the same day. 1 
These interesting facts entirely support the belief in the 
variability of all animals in all their parts and organs, to an ex- 
tent amply sufficient for natural selection to work with. We may, 
indeed, admit that these are extreme cases, and that the majority 
of species do not vary half or a quarter so much as shown in the 
examples quoted, and we shall still have ample variation for all 
purposes of specific modification. Instead of an extreme varia- 
tion in the dimensions and proportions of the various organs of 
from 10 to 25 per cent, as is here proved to occur, we may as- 
sume from 3 to 6 percent, as generally occurring in the majority 
of species ; and if we further remember that the above excessive 
variations were found by comparing a number of specimens of 
each species, varying from 50 to 150 only, we may be sure that 
the smaller variations we require must occur in considerable 
numbers among the thousands or millions of individuals of 
which all but the very rare species consist. If, therefore, we 
were to divide the population of any species into three groups 
of equal extent, with regard to any particular character — as 
length of wing, or of toes, or thickness or curvature of bill, or 
strength of markings — we should have one group in which the 
mean or average character prevailed with little variation, one in 
which the character was greatly, and one in which it was little, 
developed. If we formed our groups, not by equal numbers, 
but by equal amount of variation, we should probably find, in 
accordance with the law of averages, that the central group in 
which the mean characteristics prevailed was much more nume- 
rous than the extremes, perhaps twice, or even three times, as 
great as either of them, and forming such a series as the follow- 
ing — 10 maximum, 30 mean, 10 minimum development. In 
ordinary cases we have no reason to believe that the mean cha- 
racters or the amount of variation of a species changes materially 
from year to year or from century to century, and we may there- 
1 These facts are taken from a memoir on The Mammals and Winter 
Birds of Florida, by J. A. Allen ; forming Yol. II., No. 3, of the Bulletin 
of the Museum of Comparative Zoology at Harvard College, Cambridge, 
Massachusetts. 
