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. 110). 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 tur 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 
