size, being indeed among the largest poisonous snakes in the world—­their only rivals in this respect being the diamond rattlesnake of Florida, one of the African mambas, and the Indian hamadryad, or snake-eating cobra.  The fer-de-lance, so dreaded in Martinique, and the equally dangerous bushmaster of Guiana are included in this genus.  A dozen species are known in Brazil, the biggest one being identical with the Guiana bushmaster, and the most common one, the jararaca, being identical, or practically identical with the fer-de-lance.  The snakes of this genus, like the rattlesnakes and the Old World vipers and puff-adders, possess long poison-fangs which strike through clothes or any other human garment except stout leather.  Moreover, they are very aggressive, more so than any other snakes in the world, except possibly some of the cobras.  As, in addition, they are numerous, they are a source of really frightful danger to scantily clad men who work in the fields and forests, or who for any reason are abroad at night.

The poison of venomous serpents is not in the least uniform in its quality.  On the contrary, the natural forces—­to use a term which is vague, but which is as exact as our present-day knowledge permits—­ that have developed in so many different families of snakes these poisoned fangs have worked in two or three totally different fashions.  Unlike the vipers, the colubrine poisonous snakes have small fangs, and their poison, though on the whole even more deadly, has entirely different effects, and owes its deadliness to entirely different qualities.  Even within the same family there are wide differences.  In the jararaca an extraordinary quantity of yellow venom is spurted from the long poison-fangs.  This poison is secreted in large glands which, among vipers, give the head its peculiar ace-of-spades shape.  The rattlesnake yields a much smaller quantity of white venom, but, quantity for quantity, this white venom is more deadly.  It is the great quantity of venom injected by the long fangs of the jararaca, the bushmaster, and their fellows that renders their bite so generally fatal.  Moreover, even between these two allied genera of pit-vipers, the differences in the action of the poison are sufficiently marked to be easily recognizable, and to render the most effective anti-venomous serum for each slightly different from the other.  However, they are near enough alike to make this difference, in practice, of comparatively small consequence.  In practice the same serum can be used to neutralize the effect of either, and, as will be seen later on, the snake that is immune to one kind of venom is also immune to the other.