28 ANIMAL FORMS 



through a division of labor among the constituent cells 

 have resulted in Volvox-\ike colonies. There are the strong- 

 est reasons for the belief that as these simple forms scat- 

 tered into various surroundings and underwent changes to 

 meet the shifting conditions, they assumed different de- 

 grees of complexity that have resulted in the animal forms 

 of the present day. 



It may have been noticed also that the sponge in its 

 development passes through these stages : a single-celled 

 egg ; later, a young form similar to Pandorina, then growing 

 to look like Volvox, and finally assuming its permanent form. 

 The history of the race of sponges and their development 

 through a long line of ancestry of increasing complexity is 

 thus told by the sponge as it develops from the egg into 

 the adult ; and, so far as we know, all the many-celled ani- 

 mals in their growth from the egg repeat more or less 

 clearly the stages passed through by their forefathers. 



