NATURAL: SELECTION. ISI 
sands of the desert they frequent. The Polar Bear, living 
on ice and snow, is white or gray ; but as the summer ad- 
vances and the snow passes away, leaving the dark ground 
c 
exposed to view, the Bear changes his skin to a brown or 
ack, assuming again, as winter returns, its whitish hue. 
Mr. Darwin explains these striking facts by showing that 
the harmonizing of the color of an animal with its sur- 
roundings is useful to it. For those animals being unob- 
E 
served are favored in the struggle for existence, seizing 
more easily their prey, or escaping from their enemies more 
readily, than those not so favored. Mr. Wallace, speaking 
of the butterfly Kallima parapleta, says, “At length I was 
fortunate enough to see the exact spot where the butterfly 
settled, and, though I lost sight of it for some time, I at 
length discovered that it was close before my eyes, but 
that in its position it so closely resembled a dead leaf 
attached to a twig as almost certainly to deceive the eye 
even when gazing full upon it.” In reference to this sub- 
ject, Prof. Haeckel notices the Helmichthys, fishes whose 
bodies are so transparent that one can read a book through 
them. The Carinaria among the Mollusca, the Salpa 
among the Worms, many of the Jelly-fishes, are either bluish 
or colorless as the water they live in. The transparent 
glass-like color of these animals who live on the surface of 
the open sea is evidently of service to them in catching the 
objects of their prey or avoiding their enemies. Suppose 
now the remote ancestor of one of these animals to have 
been slightly transparent, a little more so than the indi- 
viduals of the same species, it would have been favored in 
the struggle for existence, and would have survived. Trans- 
mitting this useful peculiarity, its posterity would be still 
more transparent. Finally, in the course of generations, 
almost perfectly transparent animals would be produced. 
Prof. Cope observes, “The gray sand hue so well adapted 
for concealment is universal, with few variations, in the 
