28 
lining the beaches. The cause of this wholesale destruction 
is not a simple one. A good many factors co-operate to 
bring it about. In large measure it is the penalty of 
success. Competition for foothold is a great factor in 
algal life, and when rock surfaces are already occupied 
to full capacity, new comers must find room on the 
surfaces of previously established thalli. A second influx 
of germinating sporelings adds another storey to the 
superstructure and so on, the stability of the whole 
resting on the tenacity of the ground-floor tenant, so to 
speak. The original vegetation is eventually buried 
under an increasing load of epiphytes and, deprived of 
necessary illumination, possibly also of adequate oxygen 
supply, it becomes merely a question of time before the 
increasing leverage of moving water acting on 
a deteriorating thallus with a burden of supernumerary 
organisms proves too much for the elasticity of the 
plant tissues. Large quantities of detached weed are 
then thrown up on the shore and find a useful end 
as manure, contributing their mineral content to the 
furtherance of plant life on land. 
It would seem therefore that part at least of the autumn 
depopulation is attributable to keen competition for 
foothold on a limited area of the shore and to mutual 
interference of the components of a too profuse flora. 
Nevertheless, the foundation of this annual rise and fall of 
algal vegetation rests on the difference in physical factors 
between one season and another. 
THE FACTORS 
Fig. I is a graphic presentation of the mutual relations 
of mean monthly sea and air temperatures. It is based 
on the record of temperature data collected during twenty- 
five years, from 1903-1927, at the Marine Biological 
Station, Port Prim:. Sermtiny ofthe figure shows that 
