THE LONG ROAD 
ceeding one another, as Cope says, in the relative 
order of their zodlogical rank. Thus the sponges are 
later than the protozoa, the corals succeed the 
sponges, the sea-urchins come after the corals, the 
shell-fish follow the sea-urchins, the articulates are 
later than the shell-fish, the vertebrates are later 
than the articulates. Among the former, the am- 
phibian follows the fish, the reptile follows the am- 
phibian, the mammal follows the reptile, and non- 
placental mammals are followed by the placental. 
It almost seems as if nature hesitated whether to 
produce the mammal from the reptile or from the 
amphibian, as the mammal bears marks of both 
in its anatomy, and which was the parent stem is 
still a question. 
The heart started as a simple tube in the Lepto- 
cardi; it divides itself into two cavities in the 
fishes, into three in the reptiles, and into four in the 
birds and mammals. So the ossification of the ver- 
tebral column takes place progressively, from the 
Silurian to the middle Jurassic. 
The same ascending series of creation as a whole 
is repeated in the inception and development of 
every one of the higher animals to-day. Each one 
begins as a single cell, which soon becomes a con- 
geries of cells, which is followed by congeries of con- 
geries of cells, till the highly complex structure of the 
grown animal with all its intricate physiological 
activities and specialization of parts, is reached. It 
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