98 MOLLUSC A, INSECTS, ETC. 



to others, have no more right to complain than the 

 blood-corpuscles of our body, when they are spent in 

 energy and replaced by those newly formed ! 



The aquarium keeper soon finds that it is neces- 

 sary to be constantly replenishing his stock. Not 

 only has he first to get something like a balance of 

 animal and vegetable life, he has also to see that 

 the associated animals do not breed too fast or too 

 slowly. If the former, then he introduces one or two 

 species which keep them down by preying upon 

 them ; if the latter, he adds additional specimens. 

 If all the animals of his aquaria be carnivorous and 

 none herbivorous, his tank will soon be converted into 

 a regular field of battle, and war will be the order of 

 the day, until the combatants are reduced to the 

 fabled condition of the Kilkenny cats. He has, 

 therefore to copy nature in this respect, and mind 

 that his aquatic pets are taken from the carnivorous 

 and herbivorous classes alike, and that they are 

 placed in such a relation to each other that the 

 marvellous fecundation of the herbivorous group re- 

 places the ravages made upon them by the carni- 

 vorous. It is evident that if the herbivorous kinds 

 are in the ascendant, harm will soon issue to the 

 balance of life by the oxygen-yielding plants being 

 devoured. Hence the importance of having both 

 carnivorous and herbivorous creatures in the same 

 tank if possible. 



We have dwelt at some length upon the commoner 



