DESCENT WITH CHANGE 



361 



to speak of species as permanent entities." Invertebrates appear 

 in the Proterozoic Era; Fishes, Amphibians, and Reptiles in the 

 Paleozoic; Birds and primitive Mammals in the Mesozoic; higher 

 Mammals and Man in the Cenozoic. Mosses and Ferns arise 

 before Conifers and the latter before the familiar Seed Plants. 

 Just in proportion to the completeness of the geological record 

 is the unequivocal character of its testimony to the truth of the 



Fig. 233. — Reptilian Bird, Archaeopteryx (A), compared with Pigeon, 



Colwnba livia (B). (From Lull.) 



evolution theory. For the sake of concreteness we may select two 

 examples from the wealth of material offered by the paleontologist. 

 At first glance there seems to be little but contrasts between a 

 typical Reptile and a typical Bird; between a cold-blooded, scaly- 

 skinned Lizard, let us say, and a warm-blooded, feathered Pigeon. 

 And yet the zoologist is convinced that Birds have evolved from 

 a reptilian stock, because, in spite of superficial dissimilarities, 

 there are fundamental structural resemblances not only between 

 adult Reptiles and Birds, but also between their embryological 

 stages. And further, because the fossil remains of a very primitive 

 Bird, Archaeopteryx, have been found which form, in many ways, 



