Orga7iic Nature s Riddle 345 



In addition to the action of heredity and the action of 

 the environment, there is also a pecuKar kind of action 

 due to an internal force which has brought about so many 

 interesting cases of what is called ' serial and lateral 

 homology' which cannot be due to descent, but which 

 demonstrate the existence of an intra-organic activity, the 

 laws of which have yet to be investigated. Comparative 

 anatomy, pathology, and teratology combine to point out 

 the action of this internal force. 



' Lateral homology ' refers to the production of similar 

 structures on either side of the body, as in the similarity 

 of our right and left hands and feet. ' Serial homology ' 

 refers to the production of similar structures one behind 

 the other, as in the series of similar segments in the body 

 of a worm or a centipede, and the similar series of limbs 

 in the latter animal. 



These tendencies to lateral and serial repetition show 

 themselves in ways which cannot be accounted for by inherit- 

 ance from ancestral forms, but loudly proclaim the presence 

 and action of some internal force tending to produce such 

 homologous repetitions in organisms in different animals. 



Thus even in ourselves, when we compare our leg and 

 foot with our arm and hand, we find that they have 

 homologous features which cannot be accounted for as 

 being inheritances from supposed ancestral animals. Our 

 extremities resemble each other in the texture of the skin, 

 the shape of the nails, and other points, and these resem- 

 blances are not due to external conditions, but exist in spite 

 of them; and comparative anatomy reveals to us countless 

 similar examples in the animal kingdom. Limbs can hardly 

 be more unlike in form and position than are the arms 

 and legs of birds, and yet we meet with breeds of fowls 

 and pigeons the feet of which are furnished with what 

 are called ' boots,' that is, with long feathers which grow 



