biologic and environmental conditions, which probably are 

 peculiar for each family, or perhaps for each species. 



This is not an idle fancy, for it has been shown by 

 experimental investigations (156) on the temperature of 

 developing bird embryos that "the embryo of a chick must 

 be looked upon as a cold-blooded animal" during the early 

 days of incubation, and that later it shows more and more 

 characters of a warm-blooded animal. If this contention 

 be correct, then the fertilized egg, as such a new individual, 

 must be, in its growth, fitted, and respond, to conditions 

 peculiar to it and its immediate ancestors. In other words, 

 it must be in consonance with its environment, and react to, 

 and be affected by, it as in any other individual. The physi- 

 ology of the embryo is determined both by its heredity and 

 its environment. Would it not be logical to look for the fac- 

 tor or factors, condition or conditions, which fix the true 

 length of incubation amongst the environmental conditions 

 which make for or against the health of the developing new 

 bird? Would it not be wiser to seek for an answer to our 

 problem in the domain of the bird's physiology? 



It has already been stated that, for an egg to be success- 

 fully incubated, it must be subject, during incubation (a) to 

 a correct position, (b) to an atmosphere of proper moisture 

 content, and (c) to a certain degree of temperature. 



Tlie attainment of a correct position for the egg during 

 incubation is apparently automatically brought about by 

 the movements of the incubating parent with all birds ex- 

 cept the megapods. It seems undecided whether these sin- 

 gular birds do or do not visit or disturb an egg after it 

 is laid. 



The proper amount of moisture necessary for the preser- 

 vation of the embryo is probably attained by selection, 

 through a long process of adaptation to the surrounding 

 prevailing humidity; it would be valuable to know if eggs 

 of individuals of the same species, incubating in humid and 

 in arid regions, exhibit different egg shell structure in order 

 to compensate for humidity differences in such antipodal 

 regions. It is possible that in natural incubation the mois- 

 ture emanating from the incubating parent's body, or from 

 the soil under a ground-nesting species, also may be a factor, 

 and help combat undue dessication during incubation. It 

 is said that the Egyptian plover (Pluvianus a^gyptius), 

 whose eggs are partly incubated by the direct heat of the 

 sun, dampens its egg by contact with its previously water- 

 soaked feathers. 



The first of these two requisites seems to be one of pure 

 physics only, and the second one largely of physics, with 

 perhaps an added element of physiology. 



How important is the first in natural incubation I am 



45 



