loO MARVELS OF POND-LIFE. 



1)e expected and generally found. There is, however, 

 a curious fact about ponds, big and little, which 

 Pritchard remarks upon in his ''Infusoria," and 

 which corresponds with our own experience, that 

 those which have proved to be well stocked with 

 any particular creature during one year, will very 

 likely contain none of it in the next. There are 

 of course exceptions to this rule, but Ave have often 

 been astonished and disappointed at finding the 

 complete change, both in populousness and popula- 

 tion, that a revolution of twelve months will make; 

 and it would be extremely interesting to notice 

 the changes that took place during a term of years. 

 Such researches might unfold some unexpected laws 

 in the succession of infusorial life. Those germs 

 which are most widely diifused, will be the most 

 likely to be developed in any mass of convenient 

 water; but how and why the rarer forms come and 

 go is very imperfectly understood. Slight modifi- 

 cations in surrounding circumstances will materially 

 affect the result. Thus, if we bring home a handful 

 of conferva, and a few water-plants of higher or- 

 ganization, such as duck-weed and anacharis, and 

 place the whole in a gluss jar full of pond-water, 

 we shall at first have a good stock of objects ; but 

 they will usually grow less and less, until scarcely 

 anything is left. If, howx^ver, we introduce a few 

 pieces of straw, or a tiny wisp of hay, we shall 



