the Marine Alrjcc of the Firth of Forth. 437 



was equal to that in the former cases. Such movements 

 were, however, effected at reduced speeds without detriment 

 to the embryos. Tlie introduction of further complexities 

 in the movements only increased the retardation of growth. 



The addition of muddy material to the water, while the 

 movements were also produced, had a detrimental effect, 

 slower speeds of the motor through the water, either in 

 uniform or alternately opposite or more irregular paths, 

 producing retardation or complete cessation of growth. 



The direct bearing of such observations on the general 

 question of distribution becomes clear from the following 

 considerations : — 



1. The greater the rapidity of development of the spores 

 after emission (whether zoogonidia, zoospores, or tetraspores), 

 the better are the chances that the species will establish 

 itself to the exclusion of other competing forms in the 

 general struggle for survival, as the latter are thus forced 

 to pass either to higher or lower levels, in order to continue 

 their vitality. 



2. The heliotropic tendencies of the swarmspores, in the 

 first instance — e.g., in conditions of perfect calm — give 

 indications of the habit of the adult plant, and contribute 

 in a greater or less degree to its wider distribution. Thus, 

 positively heliotropic swarmspores may pass to a position 

 at a considerable distance from the parent, and develop on 

 any object presenting adequate conditions as regards surface 

 and depth — i.e., within a maximum and minimum of depth 

 proper to the species. In cases such as the Ulvacece, where 

 both heliotropic tendencies are manifested in succession, 

 there is (a) a movement tending to dissemination, and (b) 

 one tending to bring the former to an end, and to secure 

 the rapid and safe fixation of the embryo. 



3. The more rapid the evolution of any species, the 

 greater is the influence which it exerts in acting as a 

 resisting surface to the passage of water, bearing, it may 

 be, many other swarmspores of the same or of different 

 kinds, and the greater in consequence is its effect in 

 securing the rapid covering of the substratum on which it 

 has become fixed with other specimens of its own or other 

 species, and thereby of influencing the dynamical efficiency 

 of wave action. Similarly, too, it plays an important indirect 



