DICHOTOMY. 133 



theory it is inconceivable that these parts should accu- 

 rately adapt themselves to each other, that heads should 

 unite, hearts blend, or the alimentary canals fuse to form 

 one. On the dichotomy or cleavage theory all this 

 becomes intelligible, and the fact that reduplicated 

 digits, limbs, heads, and trunk occur throughout the 

 vertebrate sub-kingdom and beyond it, is sufficient to 

 indicate that a common cause underlies these diverse 

 malformations. 



The subject is one of great interest, for the same 

 tendency which produces dichotomy of the ray in star- 

 fish, or digits in mammals, will, when it involves the 

 axis of the limb, produce a supernumerary arm, wing, 

 or leg; should it affect the axis of the embryo, will lead 

 to the production of duplex monsters of varying develop- 

 ment and different degrees of union, or even result in 

 viable twins. The same tendency to dichotomize is 

 exhibited in the plant world. 



