BE ESIDES the birds and their immediate ancestors, only certain insects and bats have succeeded in solving the problems of true flight. Many other forms, having derived sufficient momentum from another medium, can glide through the air for a considerable distance. True flight adds to this the ability of maintaining both altitude and speed by displacement of the air itself. Much has been written of the origin of bird flight but little is definitely known. It may be that it developed from the gliding leaps of the tree-living ancestors of the modern bird. Certain amphibians and mammals incapable of true flight but capable of gliding have been observed to terminate long glides with an upward turn. This sudden gain of altitude takes them up to a selected landing place, but at the cost of their momentum. It is quite conceivable that a successful glider would lengthen the span of flights by vigorous downward displacement of the air with movements of its gliding surfaces. The more generally accepted notion, however, is that the original flyers launched themselves into the air not from treetops but from the ground. It is assumed that these forms were capable of rapid bipedal locomotion which served no doubt as a means of escape. The forelimbs being released could, in moments of need for great speed, become primitive gliding surfaces, by means of which the animal's long strides could be lengthened. This view is supported by the discovery of extinct forms (Ornithischia) which resemble this hypothetical bird ancestor. Such forms may or may not have been close to the line of descent of the birds. It is not impossible that flight was achieved independently by several different reptilian forms, and that some approached flight by one means, some by another. Thomson (z5) has compiled a list of preconditions of flight, i.e., characteristics which it is assumed an organism must have possessed before it was capable of even brief sustained flight. To this he adds a list of accessory adaptations supposed to have occurred subsequent to the beginnings of flight. Inasmuch as the exact sequence of development of such characteristics is largely a matter of speculation and since our chief interest is in bird flight as it occurs to-day, we shall here make no effort to discuss the bird's characteristics in the order of their evolutionary origin.