174 



THE DOCTRINE OF DESCENT. 



are usually very different in size and development, even 

 in those species in which the differentiation unquestion- 

 ably produced by selection has not led to polymorphism 

 or separation into personal groups performing 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 nutriment primarily furnished to 

 the single individuals is unequally and variably appor- 

 tioned according to currents 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 conveying the nutritive fluid and con- 

 necting 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 relations, 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 mu- 

 tability is slighter in lower than in higher organisms, 

 is a prejudice frequently repeated and fortified by tlie 

 old dogma of species. The doctrine of descent and se- 

 lection would fare ill if the case were so. But as the 

 shepherd unerringly knows the physiognomy of his 

 sheep where an excursionist from the town sees only 



