120 ANIMAL INDIVIDUALITY [CH. v 



defensive organs usually have a structure like that of 

 the defensive polyps in a hydroid like Hydractinia. 



There can be no doubt therefore that the various 

 " organs " do really represent modified hydroids ; and 

 as these are themselves individuals of the second 

 grade, a Siphonophoran is therefore an individual of 

 the third grade. The same process the subordina- 

 tion of the lower individualities to a higher which 

 we traced from a simple flagellate up to Volvox is 

 traceable again from Hydra to the Siphonophora : but 

 the interesting point is that nowhere else in the animal 

 kingdom is there an unbroken series a series in 

 which we can affirm positively that beginning and end 

 are close relations of such length. In the majority 

 of Siphonophora, the persons of the colony have 

 mostly only a historical individuality : some of them 

 are sometimes so much modified and reduced that it 

 has baffled all the zoologists to decide whether 

 they are homologous with individuals or with mere 

 appendages of individuals : and in function each is 

 devoted so little to itself, so wholly to serving some 

 particular need of the whole, that if one were 

 separated from the rest, it would appear a perfectly 

 useless and meaningless body to an investigator who 

 did not knoAv the whole to which it belonged. There 

 are zoologists who say it is incredible that the cells 

 of the Metazoa can be homologous with independent 

 beings like the Protozoa, impossible that a colony 



