466 RECAPITULATION. Chap. XIV. 



doubt their 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 Kecord is. Grave as these several difficulties 

 are, in my judgment they do not overthrow the theory 

 of descent with 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. 



Man does not actually produce variability; he only 



