This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
As we have seen that the lowest forms of life, both animal and vegetable, begin in the water so, in each new class of animals, there is this same grading up. Each begins as a water kind, goes up to land kinds, and then to tree kinds. Among the birds there is the duck that lives most of the time in the water. He swims more than he flies. Then there are the long-legged, long-billed birds that live most of the time on the edge of the water. There are other birds that build their nests in bushes or low trees near the water, and get their food from the seeds that grow on water plants or by catching fish or other water animals. Higher up are other birds that build their nests on the dry land in the meadow far away from the water. Others build in the bushes, higher up ; others in low trees ; still others in the tallest trees. You know how much brighter a crow in a pine tree is than a goose on a pond.
So with the frogs—water frog, toad, a kind of land frog, and a tree toad.; and even a flying frog. Notice the same thing among the rodents; the animals with sharp front teeth, like the two that first appear in baby's mouth. The beaver is a water rodent; the ground squirrel, rat and mouse are ground rodents. Then there is the tree squirrel and the flying tree-squirrel.
Water insects, moist-place insects, dry-land insects, bush insects, tree insects. Water mammals—the whale; "whales" that climb on rocks and get themselves called seals, "sea-lions" or sea-dogs; then our own home dogs. And some of these love water, like the water spaniel and Newfoundland dogs. Some are land dogs, like the fox, the wolf and greyhound. Others of this great dog-toothed, flesh-eating family—the "carnivora"-—like the bears, can climb trees. They really do climb trees to get the food that a little insect brother, the bee, gathers and makes over in its own body to feed its young just as mama bear gives milk out of her own body to feed her cubs.
How did animals and plants come to be so much like other animals and plants; and plants and animals so much like each other m shape, in their way of growing, moving and feeding and reproducing? Why does the growth of every tree keep showing us how many different kinds of things each little seed can grow into—root, bark, leaf, blossom, fruit? One answer is that all things are related to one another, branched out from the same beginning, just as great families grow into brothers and sisters, uncles and aunts; cousins and second cousins—differing more and more, as a rule, as they are more distantly related.
Another idea is that all the great families of animals—as bees and bears, birds and fish, horses and elephants—were made different in the first place, but yet made to resemble each other in these many unexpected ways to teach us how much we can learn from one another, and do for each other.
Anyhow we can all agree that living things are much more alike than we might suppose, when we know little about them, whether we agree as to just how they got to be so much alike or not.
And we can all agree, also, that it is much better to see where we are like other people in the things we believe, instead of quarrelling over the things in which we differ from them; and that, whatever else we believe, we can be sure that we are the happiest and most useful in proportion as we live to help every other body and every other thing—if we know and feel that all living things are little brothers in the water, the earth and the air.