CHAP. V.] ANALOGOUS VARIATIONS. 131 



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 profound. 

 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 comparision, the same laws appear to 

 have acted in producing the lesser differences 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 

 producing 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. Modifi- 

 cations 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 adjoining parts ; and every 

 part of the structure which can be saved without detriment will be 

 saved. Changes of structure at an early age may affect parts sub- 

 sequently 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 parts not having been closely specialised 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 variable than 

 those standing higher in the scale, and which have their whole 

 organisation 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 variable than generic cha- 

 racters, or those which have long been inherited, and have not 

 differed within this same period. In these remarks we have re- 

 ferred to special parts or organs being still variable, because they 

 have recently varied and thus come to differ ; but we have also seen 

 in the second chapter that the same principle applies to the whole 

 individual ; for in a district where many species of a genus are 

 found — that is, where there has been much former variation and 

 differentiation, or where the manufactory of new specific forms has 

 been actively at work — in that district and amongst these species. 



