314 LAWS OF VARIATION. Chap. XXV, 



Correlated Variation of Homologous Parts. — Parts which are 

 homologous tend to vary in the same manner; and this is 

 what might have been expected, for such parts are identical 

 in form and structure during an early period of embryonic 

 development, and are exposed in the egg or womb to similar 

 conditions. The symmetry, in most kinds of animals, of the 

 corresponding or homologous organs on the right and left 

 sides of the body, is the simplest case in point ; but this 

 .symmetry sometimes fails, as with rabbits having only one 

 ear. or stags with one horn, or with many-horned sheep 

 which sometimes carry an additional horn on one side of their 

 heads. With flowers which have regular corollas, all the petals 

 generally vary in the same manner, as we see in the com- 

 plicated and symmetrical pattern, on the flowers, for instance, 

 of the Chinese pink ; but with irregular flowers, though the 

 petals are of course homologous, this symmetry often fails, 

 as with the varieties of the Antirrhinum or snapdragon, or 

 that variety of the kidney-bean (Phaseolus) which has a 

 white standard-petal. 



In the Vertebrata the front and hind limbs are homologous, 

 and they tend to vary in the same manner, as we see in long 

 and short legged, or in thick and thin legged races of the 

 horse and dog. Isidore Geoffroy 4 has remarked on the ten- 

 dency of supernumerary digits in man to appear, not only on 

 the right and left sides, but on the upper and lower extremi- 

 ties. Meckel has insisted 5 that, when the muscles of the arm 

 depart in number or arrangement from their proper type, they 

 almost always imitate those of the leg ; and so conversely the 

 varying muscles of the leg imitate the normal muscles of the 

 arm. 



In several distinct breeds of the pigeon and fowl, the legs 

 and the two outer toes are heavily feathered, so that in the 

 trumpeter pigeon they appear like little wings. In the 

 feather-legged bantam the " boots" or feathers, which grow from 

 the outside of the leg and generally from the two outer toes, 

 have, according to the excellent authority of Mr. Hewitt, 6 



* ' Hist, des Anomalies,' torn. i. p. torn. i. p. G35. 

 674. 6 'The Poultry Book,' by W. B 



5 Quoted by Isid. Geoffroy, ibid., Tegetmeier, 18'5'j, p. 250. 



