{74 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 
