EVOLUTION MADE PLAIN 11 
with all animals but of the long, long route he 
and they have travelled in tlieir common de¬ 
scent from the simpler forms of life. Darwin 
says that the whole process of reproduction, 
from the first act of courtship by the male 
to the birth and nuturing of the young, is very 
much the same in all mammals. 
The lowest forms of animal life, the one- 
celled animals, are without sex, and multiply 
by dividing into half, each half developing into 
a complete cell, which in turn is subjected to 
the same dividing process. Many species of 
these lowly organisms still exist, never having 
developed further than the unicellular stage. 
All higher forms, as of fishes, reptiles, birds, 
mammals, begin life at this point—as a single 
cell. The cell, germ, or ovum of an embryo 
(the young of a mammal before birth) when 
fertilized divides, forming two cells, the two 
divide into four, the four into eight, and so on, 
until there is a colony of similar cells en- 
massed, and with a little rod of tissue—the be¬ 
ginnings of the spinal column—running through. 
The embryo at this stage is passing beyond 
the lowest grand division, the invertebrates. 
Entering the vertebrate division the embryo 
passes into the fish group. Deep grooves, gill 
slits, appear on the side of the neck of the 
embryo and six pairs of arched branches of 
arteries arise, just as in fishes, as if to give 
blood to the gills. Later all but one of these 
pairs disappear. The arms and legs of the 
child, like the legs of all other embryos, begin 
to develop, and continue to do so for some 
time, on the same plan as the fins of fishes. 
Further along in its development the embryo 
