It is the presence in the hickories of staminate flowers in clusters of threes, while in the walnuts there is but one; and by the more or less complete separation of the husks of the hickories into four pieces, while the walnuts have no such division.
Now, the pecan is allied to the walnuts by the number of its leaflets (thirteen to fifteen, while the walnuts have, in one species, seven 
Fig. 9.—Pecan (Carya olivæfarmis). to eleven, and in others fifteen to twenty-one), and by having its catkins separated at the base instead of being united. And it agrees with the hickories in having three catkins instead of one, and by the husk of the fruit separating into four pieces. The kernel is very sweet, the husk is thin, and the nut smooth and rather thin-shelled. The peculiar feature of a bitter division between the two halves of the nut is an approach to those hickories having a bitter kernel: while the hard shell and the sweet kernel ally it to the shell-bark. The tree is confined now to the States near or bordering the Mississippi River, and is thus the nearest of all the species to that spot which was once thickly clothed by its ancestors. It is noteworthy, too, that the walnuts approach the same habitat, and in the case of one species extend across the continent to California.
From an aboriginal ancestor which probably possessed a many-parted leaf, and a fruit with an entire husk and a thick shell, there came two branches. One of these, retaining the numerous leaflets, developed a nut with a corrugated shell and a thick, green husk, represented by the living species of walnuts. The other branch gave rise to a species having a nut resembling the pecan, with a smooth shell, and a husk separating more or less completely into four parts. Some of the various stages of development from such an ancestor have probably been preserved to us. The white shell-bark may be regarded as a modified descendant in which the bitter internal division has been lost, and the outer shell strengthened to afford additional protection. The mocker-nut and the thick shell-bark have acquired a still stronger covering to protect them in the same way. The small-fruited hickory is probably a stepping-stone to the pig-nut, with its thick shell and partially bitter kernel; while the thin-shelled bitter-nut and water hickory are other offshoots in which the bitter kernel does away with any necessity for a hard and thick shell.
The geographical distribution of any species or genus is an interesting and important adjunct to its history. At the present day, all the species of hickories are natives of North America east of the Rocky Mountains. Two out of four of the species of walnuts are confined to the same limits; a third is native to California and Mexico;
