express an underived, unexplained resemblance among species, have a literal meaning upon Darwin’s system, which they little suspected, namely, that of inheritance.  Varieties are the latest offshoots of the genealogical tree in “an unlineal” order; species, those of an earlier date, but of no definite distinction; genera, more ancient species, and so on.  The human races, upon this view, likewise may or may not be species according to the notions of each naturalist as to what differences are specific; but, if not species already, those races that last long enough are sure to become so.  It is only a question of time.

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.  The several subordinate groups in any class cannot be ranked in a single file, but seem rather to be clustered round points, and these round other points, and so on in almost endless cycles.  On the view that each species has been independently created, I can see no explanation of this great fact in the classification of all organic beings; but, to the best of my judgment, it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram.

“The affinities of all the beings of the same class have sometimes been represented by a great tree.  I believe this simile largely speaks the truth.  The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species.  At each period of growth all the growing twigs have tried to branch out on all sides, and overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life.  The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.  Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived