28 ANIMAL FORMS 
through a division of labor among the constituent cells 
have resulted in Volvoz-like 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 Volvoz, 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. 
