VARIATION AND MUTATION 



137 



parts vary?" some one asks. All parts vary, but some more 

 than others. 



Darwin, in Chapter V of his "Origin of Species," postulated 

 certain so-called laws of variability, which attempt to answer 

 this question, "What parts vary?" These so-called "laws" 

 which to-day would hardly 

 be dignified with the name 

 of law, are summed up by 

 Darwin at the end of this 

 chapter as follows: 



VARIATION OF 



ICTERUS BALTIMORE.20J 



Tail. * 









 



Tarsus. 







Middle Toe. 



f 



Hind, Toe. 



Bill. 



Length. 



::*!*::. 



"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 comparison, the same laws 

 appear to have acted in pro- 

 ducing the lesser differences 

 between varieties of the same 

 species, and the greater differ- 

 ences between species of the 

 same genus. Changed condi- 

 tions 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 manneV, 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 nourishment from the adjoining parts; and every part 

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



Width. 



Bill, Widl 



:::.::': 



Fro. 81. Diagram showing variation in di- 

 mensions in twenty male specimens of the 

 Baltimore oriole, Icterus galbula (formerly 

 called baltimare). (After Allen.) 



