ZooL.— Vol. III.] BANCROFT— COMPOUND ASCIDIANS. 1 75 



the zooids may differ in color ; and not the slightest tendency 

 toward an equalization of this difference could be detected. 

 The zooids changed in color in the way indicated in the 

 section on variation, but these changes did not tend to make 

 the colors of the fused colonies more similar. The con- 

 clusion, then, of this study of fusion is, that it is only behueen 

 colonies zuhich resemble one another more closely than usual 

 that fusion takes ^lace ; and that after the fusion has become 

 fully established, the tzvo colonies are in every respect like a 

 single colony, except that the zooids may be of different colors. 



V. The Question of the Individual. 



All multicellular organisms are to a certain extent colonies 

 and also to a certain extent individuals. In how far they 

 should be considered as mere aggregations or as single 

 individuals is still one of the most interesting and important 

 biological questions. The same question may be asked 

 concerning those looser aggregations of units of a higher 

 order, commonly called colonies; and it is in cases of this 

 kind that we have more hope of determining the essential 

 characters of the individual, on account of the greater 

 simplicity of the problem. Botryllus is a colony of a simple 

 type, composed of a single kind of zooids that are but loosely 

 aggregated together. On this account a discussion of the 

 results arrived at, with reference to the question of the 

 individual, will probably prove of some benefit, at least in 

 pointing out the most profitable direction for future research, 

 even if no very positive results have been attained. 



This question, then, presents itself in two forms: — 



(i) Should two fused colonies or the separated parts of 

 a single colony be considered as one or two individuals? 



(2) To what extent should 2, Botryllus colony be con- 

 sidered as a single individual and not as an aggregation of 

 zooids? What characters and activities does it possess that 

 cannot be considered as the sum of the characters and 

 activities of its zooids? 



The first question is a rather barren one, as it is in the 

 main a matter of words, and must be answered differently 



