358 BIOLOGY AND ITS MAKERS 
The geological record of other families of mammals has 
also been made out, but none so completely as that of the 
horse family. The records show that the camels were native 
in North America, and that they spread by migration from 
the land of their birth to Asia and Africa, probably crossing 
by means of land-connections which have long since become 
submerged. 
The geological record, considered as a whole, shows that 
the earlier formed animals were representatives of the lower 
groups, and that when vertebrate animals were formed, for 
a very long time only fishes were living, then amphibians, 
reptiles, birds, and finally, after immense reaches of time, 
mammals began to appear. 
Connecting Forms.—Interesting connecting forms be- 
tween large groups sometimes are found, or, if not connecting 
forms, generalized ones embracing the structural character- 
istics of two separate groups. Such a form is the archeop- 
teryx (Fig. 108), a primitive bird with reptilian anatomy, 
with teeth in its jaws, and a long, lizard-like tail covered with 
feathers, which seems to show connection between birds and 
reptiles. The wing also shows the supernumerary fingers, 
which have been suppressed in modern birds. Another sug- 
gestive type of this kind is the flying reptile or pterodactyl, 
of which a considerable number have been discovered. 
Tilustrations indicating that animals have had a common line 
of descent might be greatly multiplied. 
The Embryological Record and its Connection with Evo- 
lution.—The most interesting, as well as the most compre- 
hensive 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 de- 
