158 Heredity. 



These two cases shade into each other somewhat, hut 

 it will be convenient to treat them separately. The first 

 has just heen briefly examined, p. 156, and what follows 

 relates only to the second class of cases the variation of 

 homologous parts. 



The most familiar illustration of this law is the fact 

 that in most bilateral organisms homologous parts on 

 bo'h sides of the body tend to vary together. The law 

 holds in radially symmetrical organisms also. All the 

 petals of a regular flower generally vary in the same 

 manner, but there are many exceptions. 



The front and hind limbs of vertebrates tend to vary 

 in the same manner, as we see in long and short legged 

 or in thick and thin legged races of horses and dogs. 



It is stated that when the muscles of the arm depart 

 in number or arrangement from the 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. There are many cases where a 

 parent with extra fingers has produced a child with extra 

 toes, or the reverse, and in other cases a parent with 

 only one extra digit on one hand has had children with 

 supernumerary digits on both hands and both feet. 



In certain pigeons and fowls, especially in the trumpeter 

 pigeon, long feathers, like the primary wing feathers, 

 grow on the outside of the leg and on the two outer toes, 

 and in pigeons with the feet thus feathered the two 

 outer toes are partially connected by skin, thus showing 

 a marked anatomical resemblance to a wing. 



The various appendages which are formed from the 

 skin, such as hoofs, horns, hair, feathers, teeth, etc., are 

 homologous organs, and it is interesting to notice how 

 frequently a peculiarity in one of these structures is 

 associated with similar peculiarities in others. 



