34 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



exception of minute or microscopic organisms. Thus, it is most con- 

 venient to limit life histories to the round of growth and behavior 

 observable in nature or in culture, but this should involve, as an 

 essential feature, the use of both cjuantitative and experimental meth- 

 ods. The lack of such methods and measurements has deprived much 

 work in this field of anything more than suggestive value, and it 

 should now be generally recognized that definite and objective results 

 are rarely to be obtained without measuring factors and responses, 

 and also reactions and coactions. To what extent functional response 

 should be included is still to be determined, but the answer is affirma- 

 tive in respect to plants and sessile animals. 



It is evident that the major features of the life-cycle of plants 

 and animals are similar to the extent of passing through the round of 

 birth, growth, multiplication, senescence, and death (cf. Taylor, 1924, 

 1930, a). As has been stated, various forms and characteristics of 

 land plants are displayed by certain aquatic animals. In general 

 terms, this becomes significant in connection with the life histories of 

 the larger organisms playing a role in biotic communities. These 

 large and macroscopic multicellular organisms may for present pur- 

 poses be divided into sessile and sedentary ones, as opposed to the 

 motile types. The unicellular and microscopic species of both plants 

 and animals require special treatment, which is beyond the scope of 

 the present discussion. 



Sessile and Motile Organisms. The former includes land plants 

 generally, the large zoophytes, corals, etc., and a series of smaller 

 hydroids and the like. Most of these attached animals belong to the 

 sea or fresh water and there is also a large group of sedentary forms 

 which have little or no capacity or tendency to move about. Nearly 

 all these sessile and sedentary organisms are producers of disseminules: 

 seeds, spores, etc., are produced by plants; and eggs, free-swimming 

 young or larvae, stages in alternating generations, winter bodies, and 

 specialized parasitic stages, by animals. The disseminules are prob- 

 ably always more widely dispersed than the adults themselves, both 

 as regards space and habitat. Many come to rest more or less acci- 

 dentally in conditions not compatible with continued existence, though 

 the delicate larval stages of some animals that swim about feebly 

 before seating must not be overlooked in the matter of distributional 

 details. 



The life histories of the vast majority of these organisms may 

 be best considered as starting with the successful seating and begin- 

 ning of growth in the new position. Habitat selection by this means 

 is accompanied by an enormous loss of disseminules before seating, to 



