266 Likenesses; 07^, Philosophical A7iatomy 



correspondence, then, must be the effect of a spontaneous 

 development, and cannot be merely due to inheritance, 

 because it does not exist in other forms which, upon evolu- 

 tionary principles, are more nearly related to the hypothetical 

 root-forms. 



As to teratolog}^ it is notorious that serially homologous 

 parts tend to be similarly affected — great toes sharing abnor- 

 malities of structure with thumbs, and ankles with wrists, 

 knees with elbows, and so on. Professor Burt Wilder has 

 recorded six cases in which both the little fingers and both 

 the Httle toes were similarly affected, and one case in which 

 serial symmetry was alone exhibited, the right little finger 

 and the right little toe being the only ones affected. But 

 perhaps the most curious and instructive instances are those 

 in which the feet of pigeons or fowls are abnormally feathered, 

 or, as it is termed, furnished with 'boots.' These extra 

 feathers are developed along the very parts of the foot which 

 correspond to {i.e. are serially homologous with) those parts 

 of the bird's hand which bear the wing-feathers, so that these 

 ' boots ' are plainly a serial repetition of the true wing- 

 feathers. These foot-feathers have, indeed, been sometimes 

 known to exceed the wing-feathers in length. Moreover, 

 these foot-feathers resemble the true wing-feathers in struc- 

 ture, and are quite unlike the down which naturally clothes 

 the legs of such birds as grouse and owls. But there is 

 a more striking correspondence still, for in pigeons which 

 are thus ' booted ' the two outer digits (toes) become more or 

 less connected by skin, as is also the case with the corre- 

 sponding digits of the pigeon's hand. 



As regards pathology. Sir James Paget has declared, 

 speaking of symmetrical diseases, that 'a certain morbid 

 change of structure on one side of the body is repeated in 

 the exactly corresponding part of the opposite side ' — i.e. we 

 have a spontaneous manifestation of lateral homology. In 



