THE PRINCIPLE OF CO-OPERATION 357 



This chart shows a diphyletic tree giving the relationships within 

 the animal kingdom (Allee, 1926a). The distance from the base 

 represents the relative degree of specialization. The phyla and 

 classes underscored are those in which survival values from aggre- 

 gations have been demonstrated, other than those known to occur 

 in connection with normal sexual reproduction. Without reason- 

 able doubt proper tests would reveal that aggregations of animals 

 in all of the divisions still unchecked also possess survival value, 

 at least when the animals are exposed to unusual or unfavorable 

 conditions, such as those which would be furnished by hypotonic 

 sea-water for the marine forms or by distilled water for the Tro- 

 chelminthes. Exposure to various toxic agents would undoubtedly 

 reveal group survival, providing the group were not too large nor the 

 concentration too great. In addition to the group survival values 

 known to be so widely distributed among animals, taxonomically 

 considered, we have seen that similar survival values have been 

 demonstrated for such diverse organisms as bacteria, for the sper- 

 matozoa of several kinds of aquatic animals and of mammals, and for 

 tissue-culture cells. Evidently mutual interdependence, or automat- 

 ic co-operation, is sufficiently widespread among the animal kingdom 

 to warrant the conclusion given above that it ranks as one of the 

 fundamental qualities of animal protoplasm, and probably of proto- 

 plasm in general. 



Even if we are prepared to grant the foregoing conclusion, it does 

 not necessarily follow that the principle of automatic co-operation 

 is of great importance, though it may be exhibited by all known 

 major groups of animals. Before we can satisfy ourselves that we 

 are dealing with an important as well as a universal principle, it is 

 necessary to find how commonly it is exhibited in nature. Running 

 through the preceding pages and building up a summary of the 

 various organisms whose aggregations have been discussed in these 

 pages, we find, even when the different species of such animals as 

 planarians and grasshoppers are lumped together, about 125 such. 

 Aggregations of all these have been found in nature, with the ex- 

 ception of echinoderm larvae, which have not been reported in the 

 density in which they may be found in laboratory containers. With 

 about 14 exceptions, these aggregations are exhibited in addition 



