i89i.] 



THE BRITISH NATURALIST. 



maintained after the egg is laid, and in most birds this arrest of 

 development is normal, the bird not begining to sit as soon as she has 

 laid one egg ; but when the clutch is complete and sitting commences, 

 the heat set up recommences the development which had been 

 arrested. 



The earliest stages of development of all vertebrates are similar 

 and need not detain us, though in passing, we may point out that in 

 Sauropsidans; the proportion of nutritive to formitive yolk is high, and 

 in some birds extremely so. When we remember that all young 

 reptiles and many young birds can, as soon as hatched, feed themselves, 

 this will scarcely surprise us. The point to be strongly insisted on is 

 that a careful study of the development of young reptiles and young 

 birds reveals a closer relationship between them than between any 

 other groups of animals. The membranes which invest the 

 embryonic bird, as well as its skeleton and blood corpuscles, are most 

 reptilian in character and it is only at a comparatively late stage of 

 embryonic life that indications of the marked differences between 

 adult birds and reptiles appear. Tow^ards the latter part of the 

 hatching period, when the liver has been developed, this organ takes 

 up the nutritive yolk and converts it partly into bile and partly into 

 blood corpuscles. The large proportion of albumen in a bird's egg is 

 no doubt connected with the development of feathers which are 

 mainly built up from it ; just as a feather is the product of several 

 causes acting cumulatively during long periods of time, so the high 

 proportion of albumen in the birds" egg is a correlated result of the 

 same cumulative causes. 



As in examining the fossiliferous rocks we find that lower forms 

 have preceded higher ones, so in embryology we find indications of 

 the same progress ; if in an embryo we found traces of the characters 

 of a higher group the doctrine of descent would scarcely be tenable ; 

 if for instance a fish embryo possessed characteristics of a reptile, or 

 if a newt larva presented some mammalian features, it would be 

 difficult if not impossible to reconcile such occurrences with the theory 

 of descent, but as a matter of fact such things never happen ; at 

 various stages of its growth, the embryo presents certain features of 

 resemblance to lower forms, which w-e may regard as indications of the 

 ancestral stages through which the species has passed. These general 

 remarks are not entirely devoid of connection with the subject of this 



