innumerable branches. Natural selection, like a gardener, prunes the
tree into shape. Children might imagine that the gardener caused the
growth; but the tree would have been broader and have branched more
luxuriantly if left to itself.[A]

[Footnote A: See Nägeli, "Theorie der Abstammungslehre," p. 18;
also pp. 12, 118, 285.]

Every species must vary perpetually. Now this proposition is
apparently not in accord with fact; for some have remained unchanged
during immense periods. And natural selection, by removing the less
fit, certainly appears to contribute to progress by raising the
average of the species. The theory seems extreme and one-sided. And
yet it has done great service by calling in question the
all-sufficiency of natural selection and the modifying power of
environment, and by emphasizing, probably overmuch, the importance
of initial inherent tendency, whose value has been entirely
neglected by many evolutionists.

Lack of space compels us to leave unnoticed most of the exceedingly
valuable suggestions of Nägeli's brilliant work.

It is still less possible to do any justice in a few words to
Weismann's theory. Into its various modifications, as it has grown
from year to year, we have no time to enter. And we must confine
ourselves to his views of variation and heredity.

In studying protozoa we noticed that they reproduced by fission,
each adult individual dividing into two young ones. There is
therefore no old parent left to die. Natural death does not occur