CiiAr. V. ANALOGOUS VARIATIONS. 105 



tioii; I would almost as soon believe ■with the old and if^no- 

 rant cosinof>;'oiusts, that fossil shells had never lived, but had 

 been created in stone so as to mock the shells living on the 

 sea-shore. 



Suiiunari/. 



Our ignorance of the laws of variation is profound. Not 

 in one case out of a hundred can we pretend to assign any 

 reason why this or that part has varied, liut whenever we 

 have the means of instituting a comparison, the same laws 

 ajipear to have acted in jiroducing the lesser dillerences be- 

 tween varieties of the same species, and the greater differences 

 between species of the same genus. Changed conditions gen- 

 erally induce mere fluctuating variability, but sometimes they 

 cause direct and definite effects ; and these may become 

 strpngly marked in the coiu'se of time, though we have not 

 sulhcient evidence on this head. Habit in producing constitu- 

 tional peculiarities and use in strengthening and disuse in 

 weakening and diminishing organs, appear in many cases to 

 have been potent in their effects. Homologous parts tend to 

 vary in the same way, and homologous parts tend to cohere, 

 ilodilications in hard parts and in external parts sometimes 

 affect softer and internal parts. When one part is largely 

 developed, perhaps it tends to draw nourishment from the ad- 

 joining parts ; and every part of the structure which can be 

 saved without detriment will be saved. Changes of structure 

 at an early age may aflect parts subsefjuently developed ; and 

 many cases of correlated variation, the nature of which we are 

 unable to understand, undoubtedly occur. Multiple parts are 

 variable in number and in structure, perhaps arising from such 

 ])arts not having been closely specialized for any particular 

 function, so that their modifications have not been closely 

 checked by natural selection. It follows probably from this 

 same cause, that organic beings low in the scale are more vari- 

 able than those standing higher in the scale, and which have 

 llieir whole organization more specialized. Kudiinentary or- 

 gans, from being useless, are not regulated by natural selec- 

 tion, and hence arc varial)le. Specific characters — that is, the 

 characters which have come to differ since the several species 

 of (he same genus branched off from a common parent — are 

 more varia1)le than generics characters, or those which have 

 long becii inherited, and have not differed within this same 

 perioil. In these remarks we have referred to special parts or 



