76 EVOLUTION OF TO-DAY. 



connection between serially homologous organs. 

 Since, then, the likeness of these appendages cannot 

 be due to inheritance as commonly understood, and 

 since such cases as those just given indicate some 

 intimate internal connection between them, it is 

 evident that the theory of descent will not account 

 for serial homology. It is necessary to admit, un- 

 less some further explanation can be found, that 

 homology does not prove descent. 



One other way out of the difficulty has been sug- 

 gested, still in accordance with the theory of descent, 

 though applied in a very different line. This sug- 

 gestion, first advanced by Darwin, asks if it is not 

 possible to turn to the developing ovum for assist- 

 ance, and suppose that the internal bond of like- 

 ness may be due to the fact that the parts in 

 question are derived from similar parts of the 

 ovum. The first change which takes place in the 

 developing ovum, is its division into a number of 

 segments or cells. After this division has continued 

 for some time, until the cells are quite numerous, 

 certain cells are set apart to become developed into 

 certain parts of the adult. Now we can assume 

 that one of these cells is destined to form the four 

 appendages. This cell goes on dividing, and the 

 resulting cells are moulded into the form of the 

 legs and arms. It is easy to see, therefore, that the 

 leg and arm would show a tendency to develop in 

 similar directions, since they have descended from 

 the same cell of the embryo. Serial homology of 

 all sorts thus receives an explanation compatible 

 with that given to the more general cases. Every 



