MORPHOLOGY OF ANIMALS 



human races, are due to inheritance from common 

 ancestors. 



{a) The Cause of General Homology. General homology 

 is the result of common origin from the same substance 

 or primordium in the egg or embryo. The different fingers 

 on one hand come from a single hand-pad in the embryo 

 which partially divides into the several fingers; the same is 

 true of the teeth in each jaw, etc. The arm and leg come 

 from a common limb-ridge in the embryo which divides 

 into an anterior and a posterior portion, and the resem- 

 blance between one somite and another in the earthworm 

 is due to the incomplete division of the body into suc- 

 cessive somites. The resemblances between right and left 

 halves of the body (bilateral homology) must be traced 

 back to a still earlier period in the egg or embryo when a 

 bilateral division of embryonic substances occurs. Thus all 

 forms of general homology are attributable to origin from 

 common substances or primordia in the egg or embryo, 

 which substances undergo partial division, the same being 

 a form of incomplete asexual reproduction. 

 {b) The Cause of Special Homology must be traced back 

 to still earlier sources, namely to descent from similar 

 germ cells or germplasms. The only scientific explanation 

 of the homologies between the arm of a man and the fore- 

 leg of a horse is that both have had common ancestors. 

 Special homology is thus attributable to descent from 

 common ancestors, or to the division of a common inher- 

 itance material and its distribution to different individuals 

 in the process of sexual reproduction. It is therefore a 

 powerful argument in favor of evolution. 



