Schneider : the phenomena of symbiosis. 935 



from mere accidental symbiosis as well as from the more highly 

 specialized forms of symbiosis. 



There is a difference between the bacterial flora of the digest- 

 ive tract of man and that of the chicken or dog. Certain bac- 

 teria, which have not yet become markedly antagonistic or 

 mutualistic in their symbiotic associations, show a preference 

 for one digestive tract which indicates that there must be some 

 elective affinity. That the elective affinity is only slight is 

 evident from the fact that the bacteria referred to will very 

 readily grow and multiply upon artificial culture media, and 

 may readily be induced to change hosts. Some algae show an 

 elective affinity for certain living substrata. Sirosiphon pulvin- 

 atus occurs quite constantly upon species of Umbilicaria and 

 Gyrophora. Pleurococcus punctiformis occurs upon the young 

 thallus of Gladonia and Baeomyces, Pleurococcus vulgaris, on 

 the other hand, occurs upon the most varied substrata living 

 and dead; hence this association is evidently only accidental, as 

 the alga shows no preference for any particular host. It has, 

 perhaps, a slight preference for some of the Polyporei. 



Some of the higher crustaceans select certain corals, among 

 which they live, without forming any marked symbiotic rela- 

 tionship. In one locality (geographical area) Hydra viridis 

 seems to prefer one vegetable substratum {Nuphar), while in 

 another locality it prefers to live upon another plant, Lemna 

 polyrhiza. Some Rotifera show a preference for certain plants 

 to which they attach themselves. Certain algae, as species of 

 Dactylococcus and Euglena, show a decided tendency to locate 

 upon such animals as Cyclops, snails and clams. Some mam- 

 mals (sloth, ant-eater, and others), have algae living upon 

 them. The symbiosis of snails with corals is perhaps con- 

 tingent. Some sponges and hydroids show a preference for 

 animals, others for plants. Marine life in particular presents 

 many forms of contingent symbioses. The instances cited are 

 sufficient to indicate the nature of contingent symbiosis Many 

 require further careful study before anything definite can be 

 stated as to their biological activity and relationship to other 

 symbioses. 



II. Antagonistic Symbiosis. 



The phenomena included under this head are of wide occur- 

 rence and were the first to receive the attention of scientists. 

 The term as here used includes mutual antagonistic symbiosis 

 and antagonistic symbiosis proper. The former is not gener- 



