44 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



may keep grasses more or less permanently in the vegetative condi- 

 tion; browsing ones may in addition change the form of trees and 

 shrubs directly, or indirectly by promoting regeneration, while gall- 

 flies may completely transform shoot, leaf, or flower. It is difficult 

 and often impossible to deal adequately with the life history of 

 either plant or animal without taking their coactions into full ac- 

 count. Indeed, this many-sided interaction within the life cycle of 

 associated plants and animals constitutes the essential bond of the 

 biotic community. 



ANIMALS 



In the life histories of animals, considered from an ecological 

 viewpoint, it is the physiological states through which the animal 

 passes rather than the morphological or form stages that are impor- 

 tant. The response systems and instincts where such occur are also 

 of prime importance (cf. Taylor, 1924, 1930, a). This has led to the 

 term "physiological life history," which is unnecessary except for 

 emphasis. Again, the usual morphological procedure of starting the 

 account of the life history in the germ-plasm tissues, or with the egg 

 in the case of sexual reproduction and with the somatic tissue changes 

 in the case of asexual reproduction, must be abandoned. A logical 

 discussion of life history in motile animals usually begins with the 

 mature adult and with the primary emphasis on its response system. 

 The new motile disseminules of sessile organisms do not constitute an 

 actual beginning until seating occurs. 



Sessile and Sedentary Animals of the Waters. As has already 

 been suggested, disseminules in the form of free-swimming larvae are 

 regarded as the beginning of the life histories. These disseminules 

 are carried far and wide by currents, w^aves, and their own feeble 

 powers of locomotion. Responses to tactile, mechanical, chemical, 

 and physical stimuli largely govern attachment, but this by no means 

 insures survival to maturity. ]\Iany barnacle larvae attach to tidal 

 rock only to be killed after assuming the adult form, by the accident 

 of hot weather falling at the period of extreme low tides and conse- 

 quent maximum exposure to air (Rice, 1935). The existence of a 

 medusa stage which is both free swimming and a disseminule bearer 

 in some coelenterates does not materially alter these facts. In the 

 northern seas, there is a sequence of disseminules of associated seden- 

 tary species (Johnstone, 1908), leading to a season of maximum den- 

 sity and some seasonal groupings (Fig. 3). 



The reactions of these disseminules upon the substratum for at- 

 tachment consist in covering the surface or changing its character. 



