18 EVOLUTION MADE PLAIN 
tebrates. Here, on the invertebrate side are 
species that have the beginnings of a backbone 
—an elastic smooth cord—and gill-slits, thus 
proving their relationship with the fishes, the 
lowest of the vertebrates. 
The connecting link between the fish and 
the reptile groups is the amphibians—frogs, 
newts, salamanders, etc. The frog in its tad¬ 
pole stage is a fish, but acquiring legs and 
lungs it becomes an air-breather, a land animal. 
Between the reptile and the bird, and having 
certain characteristics of both, there are at 
least three extinct species, known by their 
fossils. One of these, archaeopteryx, had the 
skeleton and feathers of a bird and a reptile 
tail composed of twenty vertebrae. Another 
was a flying reptile with a bird-shaped head. 
Reptiles (snakes, lizards, turtles, aligators, 
etc.) are cold-blooded, egg-laying animals. Birds 
retain the egg-laying characteristic of reptiles 
but are warm-blooded like the mammals. The 
latter differ from both birds and reptiles in 
producing their young alive and suckling them. 
Between the mammal and its ancestor, the 
reptile, but classed with the former is the duck- 
mole of Australia. It is an egg-laying, web¬ 
footed, duck-billed quadruped. After its young 
are batched they are suckled in a sort of mam¬ 
mary pouch which is without nipples. Above 
the duck-mole, but so low in the mammal group 
that they are not really mammals are the 
marsupials (kangaroos, opossums, etc.). Their 
young are so immature at bir-th that they are 
for some time carried in a pouch by the mother. 
We have now come to the last great chasm— 
that between man and the other mammals. 
