240 Annals Entomological Society of America [Vol. VIII, 
normal conditions, but it is clear that growth was maintained 
because of the exchange of the voided products of respiration 
by both plant and animal. The balance was easily destroyed 
by having too many larve or too much alge. This sort of 
interrelation between organisms is very close in the forms 
studied, and therein are found the greatest number of points 
of contact with the environment than elsewhere. Any new 
factor, or one which is present but is exerting undue activity, 
stretches the relative balance which existed in the association, 
and the response caused thereby on the part of the members of 
the association will be in the direction of the establishment 
again of a new relative balance. It was quite evident from 
many observations made that in many ways the larval associa- 
tion was susceptible to quick destruction from such external 
causes as the presence of silt, wave action, etc. 
It was stated that as the dry season approached, the winds 
increased in velocity, the heat became more intense, and rainfall 
decreased. The resulting rapid evaporation concentrates the 
salts in the water of the marsh, changes thereby the density 
of the water, followed by a change in the microbiology of such 
a habitat. Such changes usually bring about less mosquito 
breeding, and on the Canal Zone for many years it has been 
observed that during the dry season Anopheles pseudopuncti- 
pennis Theob. is the dominant Anophelene—a non-transmitter 
of malaria. The dominant rainy season Anophelene is albima- 
nus. This change in the mosquito fauna is due to habitat 
changes brought about by the change in the climatic factors. 
During November and December, 1912, and January, 1913, 
there was an increase of 14.7 inches of rainfall over the same 
period the year previous. This increase of pure water at the 
swamp was sufficient to dilute the salty water to such a degree 
that subsequent evaporation did not increase the salt-content 
above a density inhibiting mosquito-breeding. This supposi- 
tion is strengthened by the fact that in previous years, condi- 
tions exactly alike excepting for this increased rainfall during 
the indicated months, this area caused no influx of mosquitos 
into Gatun. 
Reference was made to the fact that the larve from this 
swamp developed in water which equaled sea-water in chlorine 
content. The density of such water is 2.5 greater than that of 
fresh water—a difference sufficient to float chewing gum. Such 
