440 Life and Immortahty. 
Metaphorically speaking, Natural Selection may be said to 
be daily and hourly scrutinizing, throughout the world, the 
slightest variations, rejecting the bad, preserving and adding 
up the good, and silently and insensibly working, whenever 
and wherever opportunities occur, at the betterment of each 
organic being in relation to its organic and inorganic con- 
ditions of life. So slow is her work that we see nothing of 
the changes in progress, and only when the hand of time has 
marked the lapse of ages do we perceive that changes have 
been produced; but then so imperfect is our view into long- 
past geological periods, that we see only that the forms of 
life are now different from what they formerly were. That 
any great amount of modification in any point should be 
effected, a variety once formed must again, perhaps after a 
long interval of time, present individual differences of the 
same favorable character, and these must again be preserved, 
and so onward step by step. As individual differences of 
all kinds perpetually recur, this can hardly be considered as 
an unwarrantable assumption. Judged by the extent the 
hypothesis accords with and explains the general phenomena 
of nature, notwithstanding the ordinary belief that the 
amount of possible variation is a strictly-limited quantity, we 
are justified, it seems to us, in assuming that all this has 
actually taken place. But in looking at many small points 
of difference between species, which in our ignorance seem 
quite unimportant, we must not lose sight of the facts that 
climate, food and modes of life may have produced some 
direct effect, and also of the truth that, owing to the Law 
of Correlation, when one part varies, and the variations 
are accumulated through the Survival of the Fittest, other 
modications often of the most unlooked-for nature will 
ensue. 
As under domestication these variations are known to 
appear at a particular period of life, and tend to reappear in 
the offspring at the same period, so, in a state of nature, it is 
reasonable to infer that Natural Selection will be enabled to 
