310 Heredity. 



can be traced by studying their anatomy and embryol- 

 ogy, is the basis of the modern science of mor})hology. 



Now there are two kinds of homology, special homol- 

 ogy and general homology. Homologies between cor- 

 responding parts of different animals are known as 

 special homologies, and those between different parts of 

 the same animal as general homologies. As examples 

 of general homology we may instance the serial homology 

 of a cray-fish, the bilateral symmetry of mammals, and 

 the radial symmetry of a star-fish. 



So far as structure goes the homology between a man's 

 arm and a man's leg is precisely like the homology be- 

 tween his arm and a bird's wing. It is a resemblance 

 which is not due to similarity of use, but to fundamental 

 resemblance, and it is more marked in the embryo than 

 it is in the adult, and we seem, at first sight, to be justi- 

 fied in concluding that, if sjoecial homologies indicate 

 genetic descent, general homologies must also; and that 

 if general homologies cannot be explained in this way, 

 the explanation of special homologies cannot be ac- 

 cepted. 



Mivart has pointed out that it is impossible to explain 

 general homologies by attributing them to inheritance 

 from a common ancestor, and he therefore concludes 

 that special homologies do not prove genetic evolution. 

 On the other hand many authors have held that since 

 special homologies indicate descent, general homologies 

 must have the same meaning, and this belief has led to 

 such speculations as the attempt to trace the vertebrates 

 back to an annelid with a number of equivalent seg- 

 ments, to trace the echinoderms back to a community 

 of worms, and to trace the polymorphic siphonophores 

 back to unspecialized communities of hydroids. 



I hope to show in another place that the acceptance 



