202 COMPARATIVE ZOOLOGY. 
well as type. For a time there is no essential difference 
between a Fish and a Mammal: they have the same nerv- 
ous, circulatory, and digestive systems. ‘The first depart- 
ure is the alteration of the heart of the Mammal, giving 
it four cavities; while the heart of the Fish remains in its 
rudimentary condition. We may call this arrested devel- 
opment. There are many such cases, in which the em- 
bryo of an animal represents the permanent adult condi- 
tion of some lower form. In other words, the higher 
species, in the course of their development, offer like- 
nesses, or analogies, to finished lower species. The hu- 
man germ, at first, can not be distinguished from that of 
any other animal: for aught we can see, it may turn out 
a Frog or a Philosopher. The appearance of a primitive 
stripe excludes it at once from all Invertebrates. For a 
time, it assumes a structure seen only in the Fish, and 
then another, found only in Mammals. Later still, it is 
not unlike the embryos of the Ox, Dog, and Monkey sue- 
cessively ; then it looks like any other infant, and finally 
it acquires the peculiarities of the race to which it be- 
longs." All the members of a group, therefore, do not 
reach the same degree of perfection, some remaining in 
what corresponds to the immature stages of the higher 
animals. Such may be called permanently embryonic 
forms. 
Sometimes an embryo develops an organ in a rudiment- 
ary condition, which is lost or useless in the adult. Thus, 
the Greenland Whale, when grown up, has not a tooth in 
its head, while in the embryo life it has teeth in both jaws; 
unborn Calves have canines and upper incisors; and the 
female Dugong has tusks which never cut the gum. The 
“splint-bones” in the Horse’s foot are unfinished metatar- 
sals. : 
Animals differ widely in the degree of development 
reached at ovulation and at birth. The eggs of Frogs 
