The affinities of all the beings of the similar class have sometimes been represented by a great tree. one believes this simile mostly speaks the truth. The green and budding twigs may represent existing of species and those produced during every former year might represent the long succession of extinct species. At every period of growth, all the growing twigs have tried to broaden out on all sides, and overtop and kill the surrounding twigs and branches, in the same manner as species and teams of species have tried to overmaster another species in the great battle for their life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, once the tree was small, budding twigs; and this affiliation of the former and represent 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, 1,2 or 3, now grown into great branches, yet survive and bear all the other branches; so with the species that lived during long-past geological periods, only a very few now have living and modified descendants. From the initial growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera, which have now no living representatives, and which are known to us only from having been found in a fossil state.