THE GIRAFFE. 21 



than was supposed, and that " each part varies 

 to a considerable extent independently " of other 

 parts, so that " the materials constantly ready 

 for natural selection to act upon are abundant in 

 quantity and very varied in kind." While co- 

 operative parts would often be more or less 

 correlated, so that they would tend to vary to- 

 gether, coincident variation is not necessary. The 

 lengthened wing might be gained in one genera- 

 tion, and the strengthened muscle at a subse- 

 quent period ; the bird in the meanwhile drawing 

 upon its surplus energy, aided (as I would suggest) 

 by the strengthening effect of increased use in 

 the individual. Seeing that artificial selection 

 of complicated variations has modified animals 

 in many points either simultaneously or by slow 

 steps, as with otter-sheep, fancy pigeons, &c. 

 (many of the characters thus obtained being 

 clearly independent of use and disuse), natural 

 selection must be credited with similar powers, 



