46 J. B. GLELAND. 7 
their numbers in a condition of approximate equilibrium, 
at least so far as their normal habitat is concerned. They 
have had countless ages in which to multiply. If prolifie 
breeders, their population has reached that degree of mag- 
nitude capable of being sustained under normal circum- 
stances. If slow breeders, the species may be on the verge 
of extinction—and doubtless many species have become ac- 
tually extinct for this reason—or slowly diminishing, or 
holding its own, or gradually increasing, according to the 
losses taking place amongst the young before these are 
capable of reproducing their kind. Altered conditions, as 
for instance, those due to the presence of man, may vastly 
disturb this equilibrium, whilst extension more or less 
fortuitous to a new habitat may enable a rapid increase to 
occur from absence of the usual controlling factors. The 
equilibrium is therefore an unstable one, liable to swing vio- 
lently in one or other direction as the result of fire or flood, 
starvation, or an abundance of food, alteration in environ- 
ment, increase or decrease of disease, and extension to new 
habitats or encroachments on old ones. The spread of in- 
troduced animal and vegetable pests in Australia is a strik- 
ing example of the swing of the pendulum in the direction 
of phenomenal increase, due to extension of the species to 
a new habitat, and that one lacking in many of the con- 
trolling factors found in the normal surroundings of the 
species. The apparent great increase of the Australian 
blowflies, Anestellorhina augur and Pollenia stygia, re- 
sponsible now for so much blowing of living sheep, is pro- 
bably a similar example due to an altered environment, 
namely, the presence of the exotic sheep, cattle, and rabbits. 
The approaching extinction in many parts of Australia of 
the aborigine, the kangaroo and wallaby, and wombat, shows 
a swing of the pendulum in the other direction, due to an 
environment altered by the white man’s presence and ac- 
tivity. 
