present theory, as regards taxonomic standing, but within 

 the control of the temperature explanation. The incubation 

 length of the flicker, not a very high, or a small bird, is 

 given as eleven to twelve days, which seems unduly brief in 

 the light of this study. This short period {if it is correctly 

 estimated) may be due to temperature conditions, since this 

 species nests in tree holes, where there is little heat loss, 

 and lays a large complement of eggs, which, as they develop, 

 would tend to higher and higher temperatures as the em- 

 bryos grow. With other hole-nesting species laying few 

 eggs, this latter effect is not present, an absence which may 

 help to accoimt for longer incubations with these species. 



What has happened in the case of parasitic cowbirds 

 and cuckoos since their eggs are incubated by foster parents ? 

 An answer to this problem will only be had when the tem- 

 peratures of the parasitic birds and of their dupes are 

 known, as well as the temperatures of such cuckoos and 

 cowbirds given to autogenous incubating. It is not too late 

 to learn the last, as there are still some of the species of 

 these genera which incubate their own eggs. In the devel- 

 opment of this parasitic habit, it undoubtedly was at first a 

 "hit or miss" arrangement; if the parasitic eggs were given 

 too high heating by the foster parents, their embryos died, 

 and the reverse could also be true, though an incubating tem- 

 perature slightly below the optimum might result in a suc- 

 cessful, but prolonged, incubation. Such a "hit or miss" 

 method, after repeated trials through a long stretch of time, 

 together with a possible tendency of the female cowbird to 

 return, when mature, to a nest similar to that in which it was 

 raised, would eventuate in a variety of selection, which 

 would secure a large percentage of successful incubations. 

 The cuckoo is said (110) to lay eggs which are, proportion- 

 ately to itself, the smallest of all birds' eggs ; has this arisen 

 because of the possibly higher temperatures of the foster 

 parents? 



What happens when a quail's egg is hatched by a ban- 

 tam hen, or what would happen if a magpie's eggs were 

 placed under a bantam? 



It seems difficult to reconcile a mourning dove's incu- 

 bation period, which is said to be fourteen days, with that 

 of the robin's of equal length, because taxonomists are 

 agreed that the latter is a much higher bird; perhaps both 

 of these species have traveled, physiologically, equally far 

 from the pro-avis ancestor, but along quite divergent roads. 

 The ostrich is regarded, by most classifiers, as lower than the 

 emu, yet its incubation period is not as long as that of the 

 latter. This example may not be a real conflict with the 

 present "ascent theory," but the lack of concord may be due 

 to an error in taxonomy, because at least one ornithologist 



72 



