142 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



the greatest opportunity for the action of Natural Selection, 

 and the individuals that are selected appear to be those 

 possessing the greatest vigor — the ones that can absorb 

 food best and can grow fastest; both of them qualities 

 that are of the utmost importance during their later life. 



The life of the adult colonies has been observed under 

 two conditions. First, when the substratum was practi- 

 cally free from other organisms; second, when the colonies 

 had to compete with other organisms for a substratum to 

 grow on. The first state of affairs was encountered prin- 

 cipally at Woods Hole and Newport, where Botryllus is 

 usually found attached to the eel-grass, but also on the 

 under sides of rocks, floating boards, etc. On the rocks 

 and boards large colonies, equaling a hand in size, were 

 often encountered. On the eel-grass, however, and especi- 

 ally in the eel-pond at Woods Hole, where Botryllus was 

 very abundant, the colonies were much smaller. This 

 small size was undoubtedly due to the competition of the 

 large numbers of neighboring colonies; for they were so 

 crowded together on the tips of the blades of eel-grass that 

 there was no room to grow large. Furthermore, every 

 sign of vigorous competition was present. The colonies 

 were fitted accurately against each other with no space 

 between. Their area of attachment having been limited, 

 they had grown thicker than usual; especially so at the 

 edges where one colony adjoins the next. The whole 

 presented the appearance of stable equilibrium: the con- 

 testants were of equal strength, armed with the same 

 weapons, and a deadlock had ensued. 



At Naples a different spectacle presented itself. Here 

 the floats and slides were being constantly overgrown by 

 other organisms, mostly branching hydroids and bryozoa, 

 with which only a few of the strongest Botrylhis colonies 

 could successfully compete. But even in these cases 

 nothing resembling stable equilibrium was established. 

 The only salvation of the Bot?'yllus colonies seemed to 

 consist in growing rapidly and constantly conquering new 

 territory; even growing out on the branches of their 



