INTRODUCTION 3 



chiefly from Africa, called the Theriodontia ; quite sure because 

 nearly all the connecting links between the two classes have already 

 been discovered — to such an extent, indeed, that really nothing 

 distinctive of either class is left save the presence or absence of the 

 peculiar bone called the quadrate, the bone with which the lower 

 jaw articulates in birds and reptiles; and certain elemental parts 

 of the lower jaw itself. And even these bones, in certain mammal- 

 like reptiles, had become mere vestiges. Even the double condyle 

 of the mammal skull, with which the vertebrae articulate, so like 

 those of the amphibian skull that Huxley based his belief of the 

 amphibian origin of the mammals chiefly upon it, has now been 

 found in certain reptiles. Warm-bloodedness, one of the diagnostic 

 characters of birds and mammals, is not really very important, 

 since it must have arisen in these two classes independently, and 

 we may easily conceive that the earliest mammals were cold- 

 blooded or that the immediate ancestors of the mammals were 

 warm-blooded. 



It is an interesting fact in the history of the vertebrates, as of 

 all other groups of animals and plants, that the chief divisions arose 

 early in geological history. Every known order of amphibians 

 and reptiles, unless it be that including the blind-worms, was 

 differentiated by the close of the Triassic period. The frogs are 

 now known from the Jurassic. The mammals and birds also quite 

 surely date their birth from the Triassic. And this early differ- 

 entiation of the chief groups is doubtless due to the fact that the 

 potentialities of diverse evolution are limited by specialization. 

 It is apparently a law that evolution is irreversible, that it never 

 goes from the special to the general, that an organism or an organ 

 once extinct or functionally lost never reappears. And it is also 

 a law in evolution that the parts in an organism tend toward 

 reduction in number, with the fewer parts greatly specialized in 

 function, just as the most perfect human machine is that which has 

 the fewest parts, and each part most highly adapted to the special 

 function it has to subserve. And these laws explain why it is that no 

 highly specialized organism can be ancestral to others differing widely 

 from it. The more radically distinct an organism is from its allies, 

 the earlier it must have branched off from the genealogical tree. 



