H !• 



322 LAWS OF VARIATION. Chap. XXV. 



to the surprise of agriculturists, the ancient rules for judging 

 the age of an animal by the state of its teeth are no longer 



trustworthy. 3 



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 condit 



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, the petals generally vary in the same manner, 

 as we see in the same complicated and elegant pattern, on the 

 flowers 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 multiflorus) 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 Ion 



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



3 Prof. J. B. Simonds, on the Age of 4 ' Hist, des Anomalies,' torn. i. p. 674. 



the Ox, Sheep, &c, quoted in *Gard. 5 Quoted by Isid. Geoffroy, idem, 



Chronicle,' 1854, p. 588. torn. i. p. 635. 



«r 



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 extremities. cm 



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 pige on and fowl, the legs and 

 the two outer toes are heavily feathered, so that in the trum- 

 peter 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 



v *» 



f 



