174 THE DOCTRINE OF DESCENT. 



single polypes,, the cells and morphological elements. 

 The single individuals are alike in their constitution, 

 but are usually very different in size and development, 

 even in those species in which the differentiation un- 

 questionably produced by selection has not led to 

 polymorphism or separation into personal groups per- 

 forming different functions. The weal or the woe of 

 our polypes is greatly dependent on the position which 

 they occupy upon the stem ; the supply of nutri- 

 ment primarily furnished to the single individuals is 

 unequally and variably apportioned according to cur- 

 rents and tides. Hence on each polype-stem there are 

 regions where the single polypes are especially thriving, 

 others where they are just able to maintain themselves, 

 others where they cannot keep their balance. But as 

 the polype-stem is traversed by a canal system convey- 

 ing the nutritive fluid and connecting the several cells, 

 the superfluity of the well-situated cells goes to the 

 benefit of those for whom a worse lot was prepared by 

 their accidental position, and conversely. These rela- 

 tions, which, complex as they seem, are very simple for 

 our comparison, determine the form and appearance of 

 the polype-stem. Among a hundred thousand stems, no 

 two will be found absolutely alike. 



To return to the mutability of organisms, even if 

 two individuals of the same species are bred under 

 the most similar conditions imaginable, it has never 

 been possible to pronounce them absolutely alike. 

 That mutability is slighter in lower than in higher 

 organisms, is a prejudice frequently repeated and forti- 

 fied by the old dogma of species. The doctrine of 

 descent and selection would fare ill if the case were 



