On the Production of Colour in Birds' Efjfjs. 55 



The first question then is, liow could the colour have 

 been acquired ? and I do not know that anyone has 

 attempted hitherto to give any answer to it. The following 

 has occurred to me as a probable explanation of the process ; 

 at least the phenomena are referred back to piinciples 

 already recognised. 



In the first place, it is important to note that the shell of 

 the ovum is formed in the third portion of the oviduct 

 ("the uterus"), and entirely during the 12-18 hours which 

 immediately precede the expulsion or laying of the agg. 

 This is the length of the period in the case of the common 

 fowl ; we may assume, generally, a similar number of hours, 

 probably shorter, in the case of the smaller species. That the 

 formation of the shell is a process distinct from the formation 

 of the yolk, is further brought before us strikingly, by an 

 experiment of M. Tarkhanoff. He introduced a small ball 

 of amber into the upper part of the ovarium, and obtained 

 later on a quite normal Qgg, with chalazse, albumen, and 

 shell, but with the ball of amber in place of a yolk. 



At the breeding season, the females of certain animals are 

 well-known to be especially impressionable, and we think 

 that the effect of the surroundings during the time of the 

 formation of the shell, upon the mental or nervous consti- 

 tution of the bird, is a main factor in the production of 

 colour in the eo^s^s. Anv variations of value are seized on 

 by natural selection, and transmitted by the principle of 

 heredity. Individuals at the present day are influenced in 

 part by the surroundings, but mainly restricted by the 

 tribal habits of generations. We have, in fact, sufficient 

 adherence to type for an experienced collector to be tolerably 

 sure of the species of bird to which a particular e,gg belongs, 

 but sufiicient variation to make him wonder at the difierences 

 which often exist between eggs of the same clutch. As we 

 find in all groups, that some species are more stable and less 

 variable than others, so the eggs of some birds are apparently 

 fixed in colour and pattern, while those of others vary 

 within wide limits. 



We will now consider in detail, the influence of surround- 

 ings, and the utility of the eflects produced. 



The general tint of the Qgg is often protective. The 

 colour of the ground prominently before the vision of the 

 laying bird, is reproduced in various shades in the 

 eggs of the pheasants and partridges, and in our mallee 

 hen {Leipoa ocellata) and megapode. In the rich brown 



