s when our
attention is called to them that we wonder why their effects were not
understood ages ago. These elements are (1) the universal occurrence of
variation, (2) an excessive natural rate of multiplication, (3) the
struggle for existence entailed by the foregoing, (4) the consequent
elimination of the unfit and the survival of only those that are
satisfactorily adapted, and (5) the inheritance of the congenital
variations that make for success in the struggle for existence. It is true
that these elements are by no means the ultimate causes of evolution, but
their complexity does not lessen their validity and efficiency as the
immediate factors of the process.

* * * * *

Taking up the first proposition, we return to the subject of variation
that has been discussed previously for the purpose of demonstrating its
reality. The observations of every day are enough to convince us that no
two living things are ever exactly alike in all respects. The reason is
that the many details of organic structure are themselves variable, so
that an entire organism cannot be similar to another either in material or
in functional regards, while furthermore it would be impossible for an
animal to be related to environmental circumstances in the same way as
another member of its species unless it was possible for two things to
occupy the same space at the same time! Individual differences in physical
constitution are displayed by any litter of kittens, with identical
parents; it needs only a careful examination to find the variations in the
shape of the heads, the length of their tails, and in every other
character. Sometimes the differences are less evident in physical
qualities than in disposition and mental make-up, for such variations can
be found among related kittens just as surely as among the children
belonging to a single human family.

Not only do all organisms vary, but they seem to vary in somewhat similar
ways. While modern investigations have thrown much light upon the
relations between variations and their causes, of particular value in the
case of the congenital phenomena, the greatest advance since Darwin's time
consists in the demonstration by the naturalists who have employed the
laborious methods of statistical analysis that the laws according to which
differences occur are the same where-ever the facts have been examined. A
single illustration will suffice to indicate the general na