THE ABTIFICIAL INCUBATION OF OVA. 55 



the ova under the gravel as in the natural state, and 

 the plan in some instances answers well enough; hut 

 I always prefer to see how the ova are progressing, 

 as, for the want of the attention which ought to be 

 paid to them by the removal of dead eggs, &c., I have 

 seen large quantities of ova lost. It may be urged 

 that we ought to imitate nature as closely as possible, 

 but in the natural state the loss of ova and alevins is 

 enormous; if it were not, our streams would be 

 literally packed with fish, as the produce of a few 

 pairs of trout or salmon, if they all came to fry, 

 would stock almost any ordinary stream. To avoid 

 this loss, and to provide for the heavy drain caused 

 by constant fishing, we have hit upon this artificial 

 plan of incubation* and protection, and the best plan 

 is that which gives us the best results with the least 

 (reasonable) amount of trouble. If the attendant 

 had to move numberless stones every time he 

 searched for dead ova, his labour would be endless, 

 while the very act of moving the stones would 

 often destroy the ova. 



Now, the difference between the glass rods and 

 the gravel is this. The gravel, offering an infini- 

 tesimal number of salient points and angles, arrests 

 and collects the sediment which is held in. the water : 

 for there wiU be some sediment, even when the 



