We are also now quite certain that, from some of the early extinct reptiles—the immediate forbears probably of the great dinosaurs—the class of birds arose, since the structural relationships between birds and reptiles are almost as close as those between reptiles and amphibians. Huxley believed that the great class of mammals arose directly from the amphibians, and there are some zoölogists even yet who think that he was right. But paleontologists are now quite sure that they were evolved from a group of primitive reptiles, known 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 differentiation 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.