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Updated: May 10, 2025
How well the simile of a genealogical tree illustrates the main ideas of Darwin's theory the following extract from the summary of the fourth chapter shows: "It is a truly wonderful fact the wonder of which we are apt to overlook from familiarity that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes.
For when the process of classification has been carried as far as it is possible for the uncivilised to carry it when the animal kingdom has been grouped not merely into quadrupeds, birds, fishes, and insects, but each of these divided into kinds when there come to be sub-classes, in each of which the members differ only as individuals, and not specifically; it is clear that there must occur a frequent observation of objects which differ so little as to be indistinguishable.
This paleontological fact is very important, because it fully harmonises with the evolutionary succession of the Mammal orders that is deduced from their comparative anatomy and ontogeny. The latter science teaches us that the whole Mammal class divides into three main groups or sub-classes, which correspond to three successive phylogenetic stages.
"Chee!" he said indignantly. "Chee! Chee!" "No," said Calhoun. "We'll be crowded enough anyhow. We'll see her later." He nodded to one of the first four student pilots, who crisply made contact with the landing-grid office, and very efficiently supervised as the grid took the ship up. The other three of the four first-trained men explained every move to sub-classes assigned to each.
And here it becomes manifest that not only is classification carried on by grouping together in the mind things that are like; but that classes and sub-classes are formed and arranged according to the degrees of unlikeness. Things widely contrasted are alone distinguished in the lower stages of mental evolution; as may be any day observed in an infant.
But the three sub-classes differ so widely from each other, not only in the construction of the sexual organs, but in many other respects also, that we may confidently draw up the following important phylogenetic thesis: The Monodelphia or Placentals descend from the Didelphia or Marsupials; and the latter, in turn, are descended from the Monotremes or Ornithodelphia.
And manifestly in choosing the wood out of which to form his bow, the plant with which to poison his arrows, the bone from which to make his fish-hooks, he identifies them through their chief sensible properties as belonging to the general classes, wood, plant, and bone, but distinguishes them as belonging to sub-classes by virtue of certain properties in which they are unlike the rest of the general classes they belong to; and so forms genera and species.
The proposed innovation was based chiefly on three cerebral characters belonging, it was alleged, exclusively to Man and thus described in the following passages of a memoir communicated to the Linnaean Society in 1857, in which all the mammalia were divided, according to the structure of the brain, into four sub-classes, represented by the kangaroo, the beaver, the ape, and Man respectively:
It is a truly wonderful fact the wonder of which we are apt to overlook from familiarity that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes.
No, I see the trouble in the way. It could be partly disposed of in time, by sending all these sub-classes out into the other school, and organizing a new primary class out of the babies who have not yet come in." "But there would be an injustice there. It would send out many babies who ought to have the privileges of the primary-room for some time yet."
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