120 Evolution and Adaptation 
must be annually destroyed by accidental causes, which would 
not be in the least degree mitigated by certain changes of 
structure or constitution which would in other ways be bene- 
ficial to the species. But let the destruction of the adults 
be ever so heavy, if the number which can exist in any dis- 
trict be not wholly kept down by such causes, — or again 
let the destruction of eggs or seeds be so great that only a 
hundredth or a thousandth part are developed, — yet of those 
which do survive, the best adapted individuals, supposing 
that there is any variability in a favorable direction, will tend 
to propagate their kind in larger numbers than the less well 
adapted. If the numbers be wholly kept down by the causes 
just indicated, as will often have been the case, natural selec- 
tion will be powerless in certain beneficial directions ; but this 
is no valid objection to its efficiency at other times and in other 
ways; for we are far from having any reason to suppose 
that many species ever undergo modification and improve- 
ment at the same time in the same area.” 
Some of the admissions made in this paragraph have an 
important bearing on the theory of natural selection. Far 
from supposing that fortuitous destruction would have no 
influence on the course of natural selection, it can be shown 
that it would have a most disastrous effect. In many cases 
the destruction comes in the form of a catastrophe to the 
individuals, so that small differences in structure, whether 
advantageous or not, are utterly unavailing. Our experience 
shows us that a destruction of this sort is going on around 
us all the time, and accounts in large part for the way in 
which the majority of animals and plants are destroyed. 
Unless, for example, a seed happen to fall on a place suitable 
for its growth, it will perish without respect to a slight advan- 
tage it may have over other seeds of its kind. Of the thou- 
sands of eggs laid by one starfish, chance alone will decide 
whether one or another embryo is destroyed by larger animals, 
or if they escape this danger, the majority of them may be 
