THE ARGUMENT FROM EMBRYOLOGY in 



in the above instance the particular advantage gained 

 at each step, even when a complete developmental 

 series is known to us ; but in such cases our diffi- 

 culties may be largely ascribed to ignorance of the 

 particular conditions that confer advantage in the 

 struggle for existence. 



Embryonic Stages viewed as Ancestors. 



Early larval stages are of much interest, as 

 possibly indicating the forms of the earliest ances- 

 tors. The most important and fundamental point 

 is the fact that all the higher animals arise from 

 eggs, and that the bodies of the higher animals 

 are built up of cells or units, as a wall is built of 

 bricks. 



The lowest animals, or Protozoa, are single units 

 or cells, and the egg of the higher animals is also a 

 single cell. Therefore each of the higher animals 

 begins its life as a single cell — i.e., in a Protozoon 

 stage. Does not this indicate the descent of Metazoa 

 or multicellular animals from Protozoa or unicellular 

 ones ? If there is a blood-relationship between the 

 highest and lowest animals, and if the higher are 

 descended from the lower, is it not reasonable to 

 look for the origin of Metazoa in the Protozoa ? 

 May not this be the explanation of the origin of all 

 Metazoa in their actual development from single 

 cells, and may not the egg represent the Protozoon 

 stage in the ancestry ? 



Further, if animals really recapitulate, and if the 

 reason why all Metazoa begin life as single cells — 



