The Embryological Record and its Connection with Evolution.--The most interesting, as well as the most comprehensive clues bearing on the evolution of animal life are found in the various stages through which animals pass on their way from the egg to the fully formed animal. All animals above the protozoa begin their lives as single cells, and between that rudimentary condition and the adult stage every gradation of structure is exhibited. As animals develop they become successively more and more complex, and in their shifting history many rudimentary organs arise and disappear. For illustration, in the young chick, developing within the hen's egg, there appear, after three or four days of incubation, gill-slits, or openings into the throat, like the gill-openings of lower fishes. These organs belong primarily to water life, and are not of direct use to the chick. The heart and the blood-vessels at this stage are also of the fish-like type, but this condition does not last long; the gill-slits, or gill-clefts, fade away within a few days, and the arteries of the head and the neck undergo great changes long before the chick is hatched. Similar gill-clefts and similar arrangements of blood-vessels appear also very early in the development of the young rabbit, and in the development of all higher life. Except for the theory of descent, such things would remain a lasting enigma. The universal presence of gill-clefts is not to be looked on as a haphazard occurrence. They must have some meaning, and the best suggestion so far offered is that they are survivals inherited from remote ancestors. The higher animals have sprung from simpler ones, and the gill-slits, along with other rudimentary organs, have been retained in their history. It is not necessary to assume that they are inherited from adult ancestors; they are, more likely, embryonic structures still retained in the developmental history of higher animals. Such traces are like inscriptions on ancient columns--they are clues to former conditions, and, occurring in the animal series, they weigh heavily on the side of evolution. [Illustration: Fig. 108.--Fossil Remains of a Primitive Bird (Archeopteryx). From the specimen in the Berlin Museum. (After Kayser.)] An idea of the appearance of gill-clefts may be obtained from Fig. 109 showing the gill-clefts in a shark and those in the embryo of a chick and a rabbit. [Illustration: Fig. 109.--The Gill-clefts of a Shark (upper fig.) Compared with Those of the Embryonic Chick (to the left) and Rabbit.] Of a similar nature are the rudimentary teeth in the jaws of the embryo of the whalebone whale (Fig. 110). The adults have no teeth, these appearing only as transitory rudiments in the embryo. It is to be assumed that the teeth are inheritances, and that the toothless baleen whale is derived from toothed ancestors. [Illustration: Fig. 110.--The Jaws of an Embryonic Whale, Showing Rudimentary Teeth.]