27 



characters formerly regarded as peculiar to reptiles are now 

 known to occur in fossil birds, and the rudiments of others, trace- 

 able only during the embryonic stages of the living forms, 

 forcibly illustrate the progressive history of the avian race. 

 Birds also agree with reptiles in their method of reproduction, 

 for both groups are propagated by eggs hatched by external 

 heat after extrusion. The eggs of the tortoise resemble those of 

 the hen, those of the crocodilia an elongated duck's egg. The 

 microscopic structure of the shells of the eggs of birds and 

 reptiles offers slight differences which, although not sufficient for 

 systematic classification, yet afford data for the reference of fossil 

 eggs either to the reptilian or avian class. The eggs of the 

 struthious birds differ most from the rest. Those of the anomalous 

 Apteryx, which lays the largest egg relatively of any bird of its 

 size, according to Mr. P. Gervais, possess chelonian affinities, and 

 alone diverge in so marked a manner as to justify the classifica- 

 tion of that curious reptilian bird in a group by itself. It is an 

 interesting and significant fact that the neb present on the beaks 

 of young birds in the shell is also developed in many young 

 reptiles. It is used to break the shell. 



The parental and nest-constructing instinct is not equally 

 developed in all groups of birds. Most aquatic species lay their 

 eggs on the bare ground. Some land birds, like the brush 

 turkeys of Australia, cover them with leaves or decayed vegetable 

 matter, and leave them to their fate. Nor are all passerine birds 

 careful of their young, which by some species are entirely left to 

 the vicarious attentions of others. The incubating and brooding 

 instinct is also susceptible to artificial stimulation, as in the 

 case of the domestic capon, which not only hatches the eggs but 

 cherishes with anxious demonstrations of maternal solicitude the 

 young birds entrusted to its care. Some kinds of snakes hatch 

 their eggs by the heat of their own bodies. Other reptiles, like 

 the tortoise and crocodile, merely hide them in the sand, but the 

 alligators of South America are " mound builders " in their way, 

 piling over them a conical heap of dead leaves. There is, there- 

 fore, a certain amount of community in the habits of living repre- 

 sentatives of both the avian and reptilian classes. It has further 

 been in-ferred as probable that some extinct " sauropsids," the 

 flying reptiles of the Secondary epoch, "reared their young with 

 affectionate care." 



In no point do birds differ more from existing reptiles than 

 in the nature of their external covering. The hiatus between the 

 cold-blooded reptile covered with scales or skin, and the warm- 



