214 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



asexual budding or having the power to restitute the whole organism from a 

 part, than in those propagating only by sexual mechanisms. 



If we consider individuality from the functional point of view and attribute 

 it to an organism able to live and function independently as an equilibrated 

 mechanism, to which different parts of the whole contribute in a distinctive 

 manner, then we can attribute individuality to the single hydrozoon as well 

 as to a colony, in which certain constituent parts may exert different func- 

 tions. However, this seems to be a problem of minor importance, because 

 the term "individuality" is not rigidly defined ; it is used in reference to or- 

 ganisms merely in order to describe certain of their characteristics. On the 

 other hand, it may be worth while to inquire whether a connection exists 

 between the lack of a manifestation of finer differentials in these primitive 

 organisms and their great plasticity, as exemplified in their readiness to form 

 organs under the influence of external and internal environmental factors 

 and propagate asexually. All the evidence tends to the conclusion that such 

 a connection does exist, although the underlying mechanism is not yet un- 

 derstood. It is presumably also these latter characteristics which provide 

 such organisms with the potentiality of immortal life, which higher organ- 

 isms no longer possess; in the higher organisms at best, certain tissues and 

 cells may possess such a potential immortality. 



B. Transplantation and Individuality in Planarians 



In many respects conditions in planarians are very similar to those found 

 in hydrozoa. In planarians we have also to deal with a very plastic living 

 substance in which, however, again a definite state of rudimentary preformed 

 differentiation exists; within a certain range it is possible to change the po- 

 larity of organs and thus to produce heteromorphosis. However, there are in- 

 dications that in proportion to the greater differentiation which exists in 

 planarians external factors do not quite, to the same extent, influence organ 

 formation and change the polarity in these organisms as they do in Hydra. 

 In order to evaluate the relation of organ differentials and of the equilibrium 

 between the parts of an organism, on which its existence as an individual de- 

 pends, to organismal differentials, we shall also in this instance first discuss 

 very briefly the factors that determine polarity, fixity and transformability 

 of parts of the body and its various organ systems. 



The existence of a predifferentiation in this class of animals is indicated 

 by the fact that the anterior pole has a greater head-forming tendency than 

 the posterior pole. Thus, while the posterior (aboral) pole has the power 

 to regenerate a head, its ability to do so is less than that of the anterior (oral) 

 pole. Furthermore, in the region where the sex organs form, proliferation 

 in the host tissue may, according to Gebhardt, readily lead to the casting off 

 or resorption of the transplant. The existence of a rudimentary differentia- 

 tion comes out also in the specific inhibition in organ formation; thus, the 

 proximity of a head inhibits head formation (Rand, Goldsmith) and that of 

 a tail inhibits tail formation (Rand). It is also indicated by the fact that if 

 Planaria is split lengthwise into halves, each half may regenerate into a 



