ORGANIC EVOLUTION 363 



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. 



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. 



Of a similar nature are the rudimentary teeth in the jaws 

 of the embryo of the whalebone whale (Fig. no). The 

 adults have no teeth, these appearing only as transitory rudi- 

 ments in the embryo. It is to be assumed that the teeth are 

 inheritances, and that the toothless baleen whale is derived 

 from toothed ancestors. 



If we now turn to comparative anatomy, to classification, 

 and to the geographical distribution of animals, we find that 

 it is necessary to assume the doctrine of descent in order 

 to explain the observed facts; the evidence for evolution, 

 indeed, becomes cumulative. But it is not necessary, nor 

 will space permit, to give extended illustrations from these 

 various departments of biological researches. 



The Human Body. Although the broad doctrine of evo- 

 lution rests largely upon the observation of animals and plants, 

 there is naturally unusual interest as to its teaching in ref- 

 erence to the development of the human body. That the 

 human body belongs to the animal series has long been 

 admitted, and that it has arisen through a long series of 

 changes is shown from a study of its structure and develop- 

 ment. It retains marks of the scaffolding in its building. 

 The human body has the same devious course of embryonic 

 development as that of other mammals. In the course of 

 its formation gill-clefts make their appearance; the circula- 

 tion is successively that of a single-, a double-, and a four- 

 chambered heart, with blood-vessels for the gill-clefts. Time 

 and energy are consumed in building up rudimentary struc- 

 tures which are evanescent and whose presence can be best 



