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. 
