466 
EECAPITULATION. 
Chap. XIV. 
doubt tlieir weight. But it deserves especial notice 
that the more important objections relate to questions 
on which we are confessedly ignorant; nor do we know 
how ignorant we are. We do not know all the possible 
transitional gradations between the simplest and the 
most perfect organs; it cannot be pretended that we 
know all the varied means of Distribution during the 
long lapse of years, or that we know how imperfect the 
Geological Record is. Grave as these several difficulties 
are, in my judgment they do not overthrow the theory 
of descent from a few created forms with subsequent 
modification. 
Now let us turn to the other side of the argument. 
Under domestication we see much variability. This 
seems to be mainly due to the reproductive system 
being eminently susceptible to changes in the conditions 
of life ; so that this system, when not rendered impotent, 
fails to reproduce offspring exactly like the parent-form. 
Variability is governed by many complex laws,—by 
correlation of growth, by use and disuse, and by the 
direct action of the physical conditions of life. There is 
much difficulty in ascertaining how much modification 
our domestic productions have undergone ; but we may 
safely infer that the amount has been large, and that 
modifications can be inherited for long periods. As 
long as the conditions of life remain the same, we have 
reason to believe that a modification, which has already 
been inherited for many generations, may continue 
to be inherited for an almost infinite number of 
generations. On the other hand we have evidence 
that variability, when it has once come into play, 
does not wholly cease; for new varieties are still occa¬ 
sionally produced by our most anciently domesticated 
productions. 
