252 DOCTRINE OF EVOLUTION 



to remain with the others for some mutual good. The 

 biological value of the association which imposes this 

 additional obligation may be found perhaps in the fact 

 that a large group is not so readily eaten by an enemy 

 as an individual cell ; but it is clearer that the process 

 of reproduction, which consists of the fusion of small 

 '^ gametes, " or nucleated fragments produced by diverse 

 or similar parents, must be greatly facilitated by the 

 occurrence of gamete-forming individuals in one and 

 the same colony. ^^To remain together ^^ is the new 

 duty imposed by nature for the good of all and for the 

 welfare of each member of the group. Some biological 

 advantage accrues to the several components, just as 

 the banding of wolves enables the pack to accomplish 

 something which the single wolf is unable to do, al- 

 though in the latter case it is not so much a reproductive 

 alliance that is formed as an offensive and defensive 

 union. 



One step higher in the scale stands the plant-form 

 called Volvox, near the border-line betw^een the one- 

 celled and the many-celled organisms. This aquatic 

 type, about the size of the head of an ordinary pin, is a 

 hollow spherical colony, with a wall composed of closely 

 set cellular components. These elements are not all 

 alike, as in the case of colonial protozoa like Vorticella, 

 for they fall into two classes which are distinguished 

 by certain structural and functional characteristics. 

 Most of them are simple feeding individuals which 

 absorb nourishment for themselves primarily, but they 

 pass on their surplus supplies to less favored neighbors 

 if occasion demands. The other members begin life 

 like the first-named, but later they become speciahzed 



