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THE AMERICAN NATURALIST [Vol. XLVIII 



organs are used as classification characters. The ideas 

 of the reproductive organs of plants are now at the 

 " fixity" stage which on the animal side is paralleled by 

 the idea of fixed tropisms and fixed instincts, of a few 

 years since. Variability of tropisms is now well recog- 

 nized and reproductive organs in plants are being found 

 plastic, as those of animals will probably be found also. 



III. PARALLELISM BETWEEN SESSILE AND MOTILE ORGAN- 

 ISMS WITH REFERENCE TO ECOLOGY 



From a summary of the considerations above it will 

 be seen that for practical comparison the division of 

 organisms into plants and animals may be abandoned and 

 only reference to sessile and motile organisms made. We 

 may now turn to a discussion of a few general principles 

 making the division into sessile and motile organisms only. 

 The behavior of motile organisms is plastic. There 

 are innumerable cases of modification of reaction by 

 variations of physical factors (Jennings, '06; Loeb, '06; 

 Mast, '11). If for purposes of discussion we put the 

 usual 1 1 normal" reactions of motile animals over against 

 "normal" structure of sessile animals, we note that the 

 behavior response of the former parallels the structural 

 response of the latter. 



1. Breeding 



Motile Organism 

 (a) The breeding activities take 

 place within narrower limits than 

 any other activities. Merriam, 

 '90; Herriek. '02; Reighard, '08; 

 Shelford, '11a, b, c, '12a, b. 



(6) The selection of breeding 

 place and breeding activities, in- 

 cluding first activities of the 

 young, are governed by the same 

 general laws as other activities. 



Fixed (Sessile) Organisms 

 (a) Breeding and other activi- 

 ties within same limits, except that 



areas through detachability of 

 seeds and other reproductive bod- 

 ied ion of abode by sessile organ- 



havior of motile young stages or 

 through wide dissemination of non- 

 motile bodies by wind (etc.) with 

 growth under favorable conditions 

 and failure elsewhere. 



