the combined operation of numerous, diverse, and uncorrelated factors
brings about this result, and not, of course, that there is such a thing
as an uncaused event or phenomenon.

Whenever any extensive series of like organisms has been studied with
reference to the variations of a particular character, the variations
group themselves so as to be described by identical or similar curves of
error. It is certainly significant that this is true for such diverse
characters, cited at random from the lists of the literature, as the
number of ray-flowers of white daisies, the number of ribs of beech
leaves, and of the bands upon the capsules of poppies, for the shades of
color of human eyes, for the number of spines on the backs of shrimps, and
for the number of days that caterpillars feed before they turn into pupæ.

To summarize the foregoing facts, we have learned that variation is
universal throughout the living world, and that the primary factors
causing organic difference--the counterparts of human ingenuity in the
case of dead mechanisms--are the natural influences of the environment, of
organic physiological activity, and of congenital inheritance. These
factors are accorded different values in the evolution of new species, as
we may see more clearly at a later juncture, but the essential point here
is that they are not unreal, although they may not as yet be described by
science in final analytical terms.

* * * * *

We come now to the second element of the whole process of evolution,
namely, what we may call overproduction or excessive multiplication. Like
variation and so many other phenomena of nature, this is so real and
natural that it escapes our attention until science places it before us in