The outer membranes are spread out between two or more successive bronchial semi-rings, a distance from the trachea which is, in typical cases, devoid of sounding membranes; some Cuculi, Caprimulgi, and some owls.
A remnant of the chorda dorsalis and its sheath persists as the ligamentum suspensorium between the central portions of the successive vertebrae.
While the tendency is for the living forms to come into harmony with their environment and to approach the state of equilibriumby successive adjustments if the environment should happen to change, it is to be observed that the action of organisms themselves often tends to change their organisms environment.
The Arctic-Alpine sub-region consists of races of plants belonging originally to the general flora, and recruited by subsequent additrons, which have been specialized in low stature and great capacity of endurance to survive long dormant periods, sometimes even unbroken in successive years by the transitory activity of the brief summer.
There has been in effect a successive shifting of zones of vegetation southwards from the pole.