DEVELOPMENT OF BEHAVIOR 323 



or with awkward methods of moving, will be killed by their inability 

 to escape with sufficient rapidity from powerful agents. There will 

 thus be a tendency to develop a fishlike form, adapted for rapid move- 

 ments through the water; close-set, uniform cilia, and a tendency to 

 revolve on the long axis ; in other words, such characteristics as we find 

 in Paramecium. 



(2) In the second group, which reacts, like Oxytricha, by running 

 along the bottom, variations of an entirely different character will be 

 advantageous. The original cylindrical form can bring but few of its 

 cilia against a surface, and presents much resistance to the water. Varia- 

 tions in the direction of a flat form, bringing many cilia against the 

 surface, and presenting little resistance to the water as it runs along, 

 will be advantageous, and individuals with such variations will be se- 

 lected. The cilia on the surface kept against the bottom will be the all- 

 important ones, so variations in the direction of increased size, strength, 

 and rapidity of these cilia will be preserved; they will develop into 

 "cirri" and other leglike structures. The cilia on the upper side of 

 the body will be not merely useless, but a hindrance; hence they will 

 tend to be lost. The tendency to revolve on the long axis will be in- 

 jurious and will likewise tend to disappear by selection of those that 

 do not thus revolve. In this way, under the action of natural selection, 

 an organism will be developed having totally different characteristics 

 from the organisms of the first set, that react by swimming freely. It 

 will naturally approach the characteristics shown by Stylonychia, rather 

 than those of Paramecium. 



(3) On the third organism, which reacts to intense agents by secret- 

 ing a layer of mucus about itself, natural selection will act in a still 

 different manner. There will be no tendency to select rapidly moving 

 individuals, nor those having larger or more numerous cilia, nor those 

 having cilia distributed in any special way; all these characteristics 

 will indeed be disadvantageous. Spiral swimming will not be developed. 

 Those organisms that produce a thicker layer of mucus, of a more re- 

 sistant character, and do this the more rapidly, will be selected. 



(4) The fourth organism, which habitually reacts by burrowing 

 into the detritus at the bottom of the water, will be acted upon by natu- 

 ral selection in a still different way. Only those characteristics which 

 aid the burrowing will be useful and therefore selected. There will 

 be no tendency to produce a swiftly swimming organism, nor one adapted 

 to running along the bottom, nor one secreting a thick and resistant 

 layer of mucus. 



To sum up, it appears that only those variations are of advantage 

 that are used, and only such variations can be preserved by the action 



