138 CO ACTION: THE INTERRELATIONS OF ORGANISMS 



mals. The genera Empusa and Saprolegnia are known to cause epi- 

 demic diseases, the first in flies, aphids, chinchbugs, grasshoppers, etc., 

 and the second among fishfry, especially in hatcheries. A few molds, 

 such as Siiorotrichum globuliferum and Botrytis cinerea, are peculiarly 

 fatal to true bugs and many larvae, and often become epidemic on a 

 large scale during wet periods or seasons. A sac fungus, Cordyceps, 

 is not uncommon on large grubs and caterpillars, the fruits forming 

 long hornlike projections. 



The number of bacterial diseases found among insects, birds, and 

 mammals is large; some of these become epidemic and bring about 

 the destruction of flocks and herds on a vast scale. To them has been 

 ascribed the "crash," but the probability of such a happening is dis- 

 cussed in Chapter 5. 



SYMBIOSIS 



As stated earlier, the concept of symbiosis has sometimes been so 

 broadened as to become meaningless, while on the other hand it has 

 suffered from superficial observation and vivid imagination. More- 

 over, precise definition is impossible and exact application equally so 

 in the general absence of critical study. At bottom, symbiosis is 

 parasitism in some form or degree, and most so-called examples of 

 symbiosis are little or nothing else, even in such classical illustrations 

 as that of the lichen (Pound, 1892; Clements, 1897; cf. Oltmanns, 

 1923:501). The coactions involved have chiefly to do with food and, 

 since at least two organisms are concerned, have potential value for 

 the formation of colonies. When one or both are represented by a 

 single individual, no colony results, but in many cases, such as in 

 ants, the production of colonies is the rule. Among water animals, a 

 number of supposed instances of symbiosis have to do with shelter or 

 attachment, and these are found among plants as well (cf. Step, 1913). 



The following account is confined largely to an enumeration of 

 cases in which there is some evidence of mutual benefit, though often 

 slight on one side at least, and to a brief consideration of the com- 

 munity significance of the various types. These may be conveniently 

 grouped as relations between plant and plant, plant and animal, and 

 between animals. Likewise, for convenience sake, those that deal with 

 food coactions are passed in review here, regardless of whether they 

 occur in land or water biomes. 



Plant and Plant. In practically all examples of this type, the 

 parasitic nature of one of the symbionts is more or less evident, and 

 the vast majority of these are fungi or bacteria. Even in the pig- 

 mented algae, epiphytic species grade into endophytic ones, some of 



