SPECIFIC CHARACTERS HIGHLY VARIABLE 175 



mere mockery and deception ; I would almost as soon believe 

 with the old and ignorant cosmogonists, that fossil shells had 

 never lived, but had been created in stone so as to mock the 

 shells living on the sea-shore. 



Summary. — Our ignorance of the laws of variation is pro- 

 found. Not in one case out of a hundred can we pretend to 

 assign any reason why this or that part has varied. But 

 whenever we have the means of instituting a comparison, the 

 same laws appear to have acted in producing the lesser differ- 

 ences between varieties of the same species, and the greater 

 differences between species of the same genus. Changed 

 conditions generally induce mere fluctuating variability, but 

 sometimes they cause direct and definite effects; and these 

 may become strongly marked in the course of time, though 

 we have not sufficient evidence on this head. Habit in pro- 

 ducing constitutional 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 manner, and homologous parts 

 tend to cohere. Modifications in hard parts and in external 

 parts sometimes affect softer and internal parts. When one 

 part is largely developed, perhaps it tends to draw nourish- 

 ment from the adjoining parts; and every part of the struc- 

 ture v/hich can be saved without detriment will be saved. 

 Changes of structure at an early age may aft'ect parts subse- 

 quently developed; and many cases of correlated variation, 

 the nature of which we are unable to understand, undoubt- 

 edly occur. Multiple parts are variable in number and in 

 structure, perhaps arising from such parts not having been 

 closely specialised for any particular function, so that their 

 modifications have not been closely checked by natural selec- 

 tion. It follows probably from this same cause, that organic 

 beings low in the scale are more variable than those stand- 

 ing higher in the scale, and which have their whole organi- 

 sation more specialised. Rudimentary organs, from being 

 useless, are not regulated by natural selection, and hence are 

 variable. Specific characters — that is, the characters which 

 have come to differ since the several species of the same 

 genus branched off from a common parent — are more vari- 

 able than generic characters, or those which have long been 



