Herbert Spencer, in the May number of the Nineteenth Century (1886), discussing the Factors of Organic Evolution, explains the origin of the ridges in a passage which I must quote in full:—
“Continuous pressure on any portion of the surface causes absorption, while intermittent pressure causes growth: the one impeding circulation and the passage of plasma from the capillaries into the tissues, and the other aiding both. There are yet further mechanically produced effects. That the general character of the ribbed skin on the under-surfaces of the feet and inside of the hands, is directly due to friction and intermitten pressure, we have the proofs: first, that the tracks most exposed to rough usage are the most ribbed; second, that the insides of hands subject to unusual amounts of rough usage, as those of sailors, are strongly ribbed all over; and third, that in hands which are very little used, the parts commonly ribbed become quite smooth.”
CHAPTER III
FINGER-PRINT PATTERNS
Before reading this chapter, let the reader carefully examine the clear lineations shown so well in the photographic picture of the Zebra’s stripes, opposite. They will be found to resemble very closely the lineations on the skin of human fingers, as printed when enlarged by photography, forming very similar patterns. Similar linings occur in the hide of the tiger.
Grevy’s Zebra.—Showing Lineations like Finger-Print Patterns.