THE SUN-PERCH AND THE CATFISH 



simply to send for foreign species and acclimatize them; 

 we must also make sure that the new-comers do not 

 enter into competition with our native species, which 

 are often worth more to us than the species so introduced. 

 The correct amount of food is on the table, the equi- 

 librium is settled, and when this is so, the new-comers 

 are intruders and are obliged to fight as best they can 

 in order to maintain themselves and live. They are 

 subject to all the changes and chances of the conflict of 

 which they are the cause. Some go under, others 

 flourish. Those which do not become acclimatized, 

 disappear, for they do not find, either as regards 

 quantity or quality, the materials they must have. On 



Fig. 44. — Catfish. Usually about a foot long. 



the other hand, the sun-perch survives, because its 

 small size sets a limit to its requirements and enables 

 it to find with us all it needs. And as for the catfish, 

 which is more robust than our native fish, it develops 

 and advances more than we care for. 



" But for every evil there is a cure, so long as we 

 know its exact cause. The breeders of carp, in their 

 large ponds, now know that their success is proportion- 

 ate to the nutritive capacity of their waters ; this must 

 be improved if the best results are to be obtained. 

 Rivers and streams are more difficult, for in their case 

 it is not easy to ensure a continuous improvement in 

 this respect. Yet, by observing Nature's rules and 

 thanks to the co-ordination she has instituted, we have 

 the power to get results up to a point. Before repopu- 

 lating by putting in fry, we must first study the degree 

 206 



