THE ORIGIN OF SPECIES 359 



Even a casual survey of this history natural history 

 of the Earth and its inhabitants cannot but impress one with 

 the fact that, taken all in all, there has been a continuous, 

 though not always a uniform, advance in the complexity of 

 organisms from the most ancient times, and that the older 

 types seem gradually to melt into modern forms as the 

 remoter geological eras merge into the more recent. " Only 

 the shortness of human life allows us to speak of species as 

 permanent entities." Invertebrates appear in the Protero- 

 zoic Era; Fishes and Amphibia in the Paleozoic; Reptiles, 

 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 Flowering 

 Plants. "Just in proportion to the completeness of the 

 geological record is the unequivocal character of its testimony 

 to the truth of the evolutionary 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 con- 

 vinced that Birds have evolved from a reptilian stock, 

 because, in spite of superficial dissimilarities, there are funda- 

 mental structural resemblances not only between adult 

 Reptiles and Birds, but also between their developmental 

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

 primitive Bird, Archaeopteryx, have been found which form, 

 in many ways, a connecting link between the Reptiles and 

 Birds as we know them to-day. 



Archaeopteryx was undoubtedly a bird about the size of 

 a Pigeon, but one with jaws supplied with many small teeth; 

 with a long lizard-like tail formed of many vertebrae, each 



