1902.] NATURAL SCIENCES OF PHILADELPHIA. IBo 



differ structurally! In the Tunicate organization itself a great 

 series of " individuals " may be distinguished, as one will. Thus 



(1) a cellular individual, such as a chromosome or a centrosome; 



(2) a tissue individual, a cell; (3) an organ individual, a tissue 

 or some specialized part of the organ ; (4) what is generally spoken 

 of as an organ, such as the heart or the intestine; (5) an organ 

 system, such as the body wall; (6) what is generally regarded as 

 the individual in this particular case, and (7) what is termed the 

 corm. For the cytologist, the embryologist and the anatomist th^ 

 idea of what constitutes the individual will differ. Likewise, there 

 are many possibilities of interpretation of the organization of a 

 Siphonophore ; and in the ease of a proliferating Hydra or Mlcro- 

 stomum, it is practically impossible to decide at just what time two 

 individuals appear in the place of the original one. 



All the preceding bears out what each naturalist finds in his own 

 experience, that there occur many degi'ees of individuals, and that 

 a sharp definition to cover all cases is practically impossible. Per- 

 haps the nearest to it is foimd in the idea of structural disassociatiou, 

 though this too breaks down when we consider the various kinds of 

 conns and colonies found in organisms, and 1 he cases of intimate sym- 

 biosis (such as the Turbellarian Convoluta with its enclosed plant 

 cells). And yet to work upon our phyletic classification, it is 

 necessary to know Avhat are the individuals which should be the 

 materials of the study ; perhaps they may be more closely deter- 

 mined by the following line of reasoning. 



Progressive evolution is a change from the more simple and 

 generalized to the more complex and specialized in both the indi- 

 vidual and the race. In the evolution of the race we consider the 

 organism as a Avhole. This gives us the working criterion of indi- 

 vidual, which is concisely the largest disassociated whole. And it 

 is in this sense that the term individual is generally employed hy 

 phylogeuists — that is, " individuals" are the units which make up 

 our concept of "species," as the latter term is usually employed 

 by naturalists. This definition will not apply to all cases, as, for 

 instance, certain corms; but in many, if not all, such cases the 

 ' ' individual ' ' and the ' ' colony of individuals ' ' may be eventually 

 distinguished by comparison of undoubted single individuals with 

 corms of imited individuals of genetic affinity with the former. 

 And it is necessary wherever possible to distinguish an individual 

 from a corm, so that relatively similar units may be compared. 



