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90 THE CONDOR Vol. XIX living birds the body temperature is at an average of from 5 to 10 degrees lower than in the more highly specialized forms such as our songbirds, where it may reach 111 degrees F. With this increase in temperature the guarding of the nest by the parents became more and more a definite act of incubation in which the heat was furnished by the body of the parent. Thus the hatching was speeded up and remo;ed from the uncertainties of weather or the irregularities of decom- posing vegetable matter. It is interesting to note at this point that the Mega- podes, which leave their eggs to be incubated by the warm sand or by decayins vegetable matter, of which group I have already mentioned the Australian Brush Turkey, nearly all occupy islands in the Australian region where there are few or no small predaceous mammals. This style of nesting would be of little protec- tion against such enemies, for the mounds are conspicuous, the eggs are rela- tively large, and incubation is slow. The heat is low and the young have to be advanced enough at hatching to burrow out of the sand or mass of debris and Fig. 37. a. CIRCULATORY SYSTE? IN TI?E ANTERIOR END OF A REPTILE, WITI? TI?E ?VIIXED VENOUS AND ARTE- RIAL RLOOD RATI?IING TI?IE RRAIN. ?. CIRCULATORY SYSTEM IN TI?IE ANTERIOR END OF A BIRD? IN WI?IICI?I ONLY FRE S I?I ARTERIAL RLOOD REACI?ES TttE RRAIN. xvardly from limb to limb, flight hustle a living for themselves at once. We might speculate that this simple form of nesting and those birds that practiced it have been eliminated in other parts of the world by some such epemy. 4. A corollary to this increase in body temperature is the development of the rep- tilian scales as a whole or in part into the delicate fimbriated scales of birds which we know as feathers. These made a high body heat possible, and made the heat for incuba- tion more reliable, as they helped cover the eggs. Incidentally they made flight possible in this group, and with flight the high devel- opment of arboreal nests so characteristic of the more specialized forms. Birds were probably evolved from the bipedal Dinosaurs and came into existence without that ability to climb which is the birthright of the small quadrupeds, where the possession of both fore and hind feet in a generalized form mechanically fits the owner for an arboreal life. Instead birds probably hopped awk- at first being merely a lengthening of such leaps. The seansorial birds, such as woodpeckers, creepers, nuthatches, certain parrots and others, are highly specialized groups in which the climbing habit has been only recently redeveloped. The preceding factors are especially associated with the actual incubation of the eggs. The succeeding are more immediately associated' with the art of nest building itself. 5. Coming at this same time of transition, when the birds were differenti- ated from their reptilian ancestors, was the change in the heart from a three- chambered to a perfect four-chambered organ. ,Thus in the anterior end of the body were the venous and arterial bloods entirely separated. This great advance in the circulatory system meant that only the freshest and purest blood went to the brain, the delicate eelIs of this organ being stimulated and nourished by the