146 TEREDIXID.E. 



wherever this was the case the ship- worm got in, and 

 speedily reduced the thickness or strength of the plank 

 to little more than that of an egg-sheU. I have not un- 

 frequently noticed crowds of very young individuals in 

 a small and thin strip of deal, which could not accom- 

 modate any one of them if it grew larger : in fact each 

 had gone to the very end of its tether; and another step 

 would have laid bare its foot, and thus have exposed the 

 most \Tilnerable part of the body to its rapacious enemies. 

 Not having room to grow, or the power of removing to 

 a larger piece of wood, all these individuals must neces- 

 sarily perish without arriving at maturity. This fact 

 apparently illustrates a law of nature, which might be 

 termed blind ; but it may also be regarded as one of the 

 numerous methods by which various races of animals 

 and plants are kept under, so as to prevent an excessive 

 multiphcation of any of them to the exclusion or detri- 

 ment of the rest. If no such checks were imposed, aU 

 the wood on the face of the earth, if placed in the sea, 

 would probably not suffice to contain the Teredines 

 produced in a single year. The natural span of life 

 allotted to the Teredo is unkno^vn to us : perhaps it may 

 be ascertained by means of the aquarium. It is supposed 

 that they attain their full growth in the course of a 

 few months. Extreme cold is fatal to them. Accord- 

 ing to the observations of Quatrefages on the north 

 coast of Spain, nearly all appear to perish in the winter ; 

 a few only survive to continue the breed. Vrolik be- 

 lieved that they hybernate on the Dutch coast. Warm 

 and dry seasons are favourable to them. In Holland, 

 where their proceedings have been watched with so 

 much anxiety, it was noticed that the greatest ravages 

 are made in July and August, and that the most de- 

 structive years during the last and present centuries were 



