PROGRESSION. 21 
When alarmed, snakes hiss with the object of frightening off 
their enemies. If snakes were not able to make their presence 
known, they would be frequently trodden upon and injured by 
the various creatures of veld, forest, and mountain. For long 
ages the wild animals have, from bitter experience, learned 
instinctively to avoid snakes. The instinct has been so strongly 
developed that it is transmitted from parent to offspring. This 
instinct persists in a greater or lesser degree, even in domesti- 
cated animals whose ancestors, perhaps for scores of generations, 
have never seen a snake. 
An animal will often manifest the wildest alarm at sight of 
a snake, or on hearing a snake hiss, although it has never before 
come in contact with one of these reptiles. This power of trans- 
mitting experiences is a wonderful provision of Nature, for if 
every living creature had to learn entirely by personal experience 
to avoid its enemies, and find out what kinds of foods were whole- 
some, or the reverse, a great number of species would rapidly 
become extinct. Venomous snakes invariably give a warning 
hiss, if suddenly and unexpectedly approached or trodden upon. 
If the latter should occur, they instinctively bite. 
Nature has therefore evolved the power of hissing as a means 
of protection to the snake, and as a warning to other creatures 
that a reptile capable of inflicting serious mischief upon them is 
in dangerous proximity. 
PROGRESSION. 
Snakes progress or crawl about by means of their ribs and 
belly scales. The backbone of a snake is wonderfully constructed, 
and is capable of being twisted and curved in a great variety of 
ways without fear of dislocation. There are a great many joints 
to the backbone, often as many as three hundred. To each of 
these sections a pair of ribs are attached, one on each side. The 
heads of the ribs are loosely attached to the backbone sections, 
and they can thus be moved forwards and backwards with the 
greatest of ease. The extremities of the ribs are attached to the 
scales or horny shield-plates of the abdomen. When the snake 
crawls, the ribs along one side of its body move forward, the edges 
of the scales gripping the rough surfaces of the ground, tree, or 
whatever the snake may be crawling over. Then the ribs on the 
