124 HABITUDES AND ATTITUDES 



smallest species yield the smallest eggs and the 

 largest birds lay the largest eggs, but the inter- 

 vening sizes show great fluctuations, as may be 

 seen by comparing the egg of a Megapode with 

 that of a Jungle Fowl. 



Amongst reptiles there are many exceptions 

 to the rule of oviparity, but there is no known 

 correlation between lecithality, or the relative 

 quantity of yolk in the egg, and viviparity ; this 

 latter habit not leading to a reduction of the 

 vitellus, although it may affect the shell-formation, 

 causing the complete absence of calcareous de- 

 posits, so that the embryo shows clearly through 

 its thin transparent envelopes. 1 The intra-uterine 

 development of a yolk -laden egg is a special 

 phase of incubation or brood - nursing, and has 

 been assumed independently by some arboreal 

 and deserticolous lizards of diverse families in 

 distant parts of the world. 



A fundamental rule with regard to methods 

 of propagation is that oviposition preceded vivi- 

 parity, but this rule does not assist in determining 

 whether the ancestors of a particular class were 

 egg-layers or vivipars, and consequently whether 

 the egg-laying members of a group where vivi- 

 parity predominates are primitive in that respect. 

 Lecithality, oviparity, and viviparity are or may 

 be independent phenomena. It is probable, for 



1 Cf. A. Willey, 1906 , " Viviparity of Cophotis ceylanica and 

 Oviparity of Ceratophora stoddartii" Spolia Zeylanica, vol. iii., 

 pp. 235-237 and figure. 



