298 ^Ir. Percy R. Lowe : 



density of tlie vegetation on the Sontli American continent, 

 leading to such general conditions that it was more profitable, 

 or indeed compulsory, for the Guan and its congeners to 

 tt'ni[)orarily forsake their arboreal retreats and to descend 

 to the perils of the ground. 



To mv mind it seems more probable that this attempt to 

 descend has only taken place in comparatively recent times, 

 since tlic davs when wc know that vegetation lias been 

 generally tending to diminisb. A further possible expla- 

 nation, which is, of course, purely hypothetical, of this more 

 recent tendency to deseeiul to the ground might be sought 

 ill some danger to the cbick in tlie trees, such as a gi-adual 

 increase at some past epoch in the prevalence of tree-snakes. 

 I bave on several occasions seen tree-snakes attacking birds, 

 and anyone who has watcbcd a pair of })arent birds noisily 

 defending their offspring from tlie slow and deadly advance 

 of these murderous assailants would allow that such a con- 

 dition of things as I have suggested migiit quite possibly 

 have been enough to start the process of a descent to the 

 ground. 



Other facts which occur to me as being against reversion 

 are : — 



(1) The white (non-protective) coloration of the Guan's 

 egg seems rather to point away from a previous terrestrial 

 existence"^, for " It is almost certain," says Mr. Pycraft in 

 his ' History of Birds,' p. 207, "that the eggs of the earliest 

 birds were Mliite, like those of their forebears the reptiles ; 

 and further, since these primitive birds were arboreal, that 

 they were laid in boles of trees or under cover. Later, when 

 some migrated from the forest region to the plains or 

 meadows, colour became nccessaiT ; firstly for protective 

 purposes, and secondly, probably, as a defence against the 

 action of light, which in excess is inimical to protoplasm." 



But granting that this is true, which seems probable, then 

 those who hold that the arboreal existence now led by the 

 Ciuaiis (Ortii/'/s) is an instance of reversion must also agree 



* Tree-1'artridges, ou the other luuid, also lav white eggs. 



