34 T^he Economy of Nature. [zoE 



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produced by the respiration of the fish and using it for its own 

 growth. 



The globe that contains fish without watercress soon must have 

 its water renewed or the fish will die by gradual suffocation. 



Something similar will take place if watercress is placed in a third 

 globe without fish. The watercress will wither amid a slimy matter. 

 This slimy matter is, it is true, also of vegetable origin. It belongs 

 to the group of Thallophytes whose biological conditions do not 

 require a considerable amount of carbon, and which adapt them- 

 selves very well to the surplus of oxygen that, during the short 

 time the watercress sustains its biological processes, is secreted by 

 Its growth. If the first globe is exposed to the sunlight, the whole- 

 some state of the water, and with it the equilibrium between animal 

 and plant life, may be kept up for a considerable time without re- 

 newing the water. 



We see that the existence of both primary^ forms of organic life is 

 a biological law. But now a great many complications arise by a 

 network of cause and eflfect that connects not only the two fundamen- 

 tally different types of organic lite, that is, animal and plant, but 

 also animal and animal, plant and plant. To give an idea of the 

 long chain of causes that produce certain phenomena I will first re- 

 mind you of the phenomenon quoted by that most talented ob- 

 er, Darwin, when in a certain district the destruction of cats 

 caused the disappearance of the wild pansies occupying before a 

 prominent place among the agricultural weeds of the locality. The 

 field mice muldplied considerably; they ate the young brood of the 

 hornet and bumble bee and destroyed their nests. Now the pansy 

 belongs to those plants ihat rarely, or never, produce perfect seeds 

 without the agency of these insects in carrying the pollen to the 

 stigma of the flower. No new plants sprang up, the fecundatino- 

 msects having disappeared, and the old plants gradually died out! 



Now you see the removal of the cat caused the dying out of the 

 pansy. 



I can give here an instance of a similar concatenation of efforts 

 that caused the defoliation of our live oak, Oicercus agri/oHa 

 There exists around our bay a moth, Phryganidia Calijornica 

 which moth lives exclusively on live oak, thoughl lately have found 

 some stray larvae on Q. lobaia. When, in 1853, I first found the 

 caterpillar of this species, I considered it a great prize, so rare was 



