VARIATION AND MUTATION 



1 '^' 



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

 than others. 



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

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

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



which to-day would hardly 



VARIATION OF 



ICTERUS BALTIM0RE.20.^ 



be dignified with the name 

 of law, are summed up by 

 Darwin at the end of this 

 chapter as follows: 



Tail. 

 • 4 



• • 



wing. 



• !•••••• 



•;r 



Tarsus. 



••••• • 



Middle Toe. 



• ••• • • 



• ••• f • • • 



/y//?d| Toe. 



i*. 



"Oiu- 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 i)ro- 

 ducing constitutional peculiarities, and use in strengthening, and 

 disuse in weakening and diminishing organs, ajipear in many ca.«ii'S 

 o have been potent in their effects. Homologous j^arts tend to 

 vary in the same manner, and homologous parts tend to coIum-c. 

 Modifications in hard parts and in external ])arts sometimes affect 

 softer and internal parts. When one part is largely develojxMl, ]>erhaps 

 it tends to draw nourishment from the adjoinin*; parts: and every ])art 

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



Bill, Length. 

 Bill, Width. 



• ••••{•• • • 



Fig. 81. — Diagr.am showing variation in di- 

 mensions in twenty male specimens of the 

 Baltimore oriole, Icterus galhula (formerly 

 called haltimore). (.\fter .\llen.) 



