538 



BOA. 



are also readily distinguished by the markings on the 

 upper part, which consist of a double row of round 

 black spots down the sides of the mesial line, with 

 eye-shaped spots on the flanks. The general colour 

 is brown. The species which forms this section is 

 Boa set/tali of Linnaeus, and Boa aquatica of Prince 

 Maximilian. It is much more, aquatic than the for- 

 mer ; and while that watches for its prey in the thick 

 and tangled parts of the woods, this waits more fre- 

 quently in the waters, and seizes and swallows the 

 animals which resort there to drink. 



Boa Constrictor. 



A third section have scaly plates on the muzzle, 

 and those on the sides of the jaws furrowed or 

 dimpled. The species which forms this section is the 

 speckled boa (Boa cenchris, Linnaeus); its ground 

 colour is yellow, marked along the back with a suc- 

 cession of brown rings, and with variously formed 

 and coloured spots along the flanks. 



These three species or sections form the true or 

 characteristic boas. They all attain nearly the same 

 size, and do not differ much in their habits. They 

 all frequent places which are humid, although some 

 inhabit further into the marsh, or are more frequently 

 found in the water than others. They are often 

 found with the prehensile tail curled round a tree by 

 the side of a pool or stream, waiting for their victims, 

 and at other times they are stretched along the water, 

 in which state they bear some resemblance to floating 

 sticks. They are not however, very often seen, 

 because the places to which they resort are very un- 

 wholesome, and not very tempting in other respects. 



The time of their greatest activity is said to be 

 during the subsiding of the floods ai'ter the rains, a 

 season at which the places which are left bare are 

 peculiarly pestilent ; but the different species of living 

 creatures to which these places are native, whether 

 animal or vegetable, are then in a state of energy 

 and activity, unknown in more open and healthy 

 places. Indeed it is the very rankness of these places 

 which makes them so rich, their pestilent qualities, 

 which enable them in the course of a few months, or 

 even weeks, to send forth a vegetation and produce 

 an assemblage of living creatures which would be the 

 work of years for the more sober (but to man more 

 healthy) powers of life in temperate climates. 



As the hot season advances, the action of the fiun 

 begins to destroy what 1t was so powerful in pro- 

 ducing while the ground was humid. This of course 

 takes place first at the higher level of the inundation, 

 where also the productiveness of the former part of 

 the season commenced ; and it gradually creeps on- 

 ward, as the water dries up and the vegetation withers, 

 toward the channel of the river whose overflow pro- 

 duced the inundation, from which this excess of living 

 action arose. These, in many places the productive 

 surfaces, at least in so far a* seasonal plants and sea- 

 sonal animals are concerned, is reduced to a mere 

 line, and the food of these large serpents becomes 

 scarce, and not easy to be had. But they are 

 tempered to all the changes of climate of which they 

 are so exclusively and peculiarly native ; and the 

 excess of heat and drought begins to have au effect 

 upon them not altogether dissimilar to that which 

 they experience when they are carried into colder 

 climates. 



It appears to be a general law of nature, and it is 

 one the knowledge of which is of much practical 

 utility, that if any animal can bear with difficulty a 

 change from its natural degree of heat in the one 

 direction, it can bear it just as ill in the other. The 

 power of endurance appears to extend from what 

 may be called the mean natural temperature, about 

 equally toward cold and toward heat. We (ind that 

 those animals which hybernate during the cold months 

 in high latitudes are most oppressed by heat when 

 the weather is very hot. We have examples of this 

 in the case of moderately low average temperature, 

 in bears, marmots, and bats, which though they 

 become languid or wholly dormant during the rigour of 

 the winter, feel very uncomfortable in the heat of the 

 day during summer ; so much so, that they generally 

 keep in concealment then, and come abroad only 

 when the ?un is low, or in the twilight. We have an 

 instance of it in a case of much lower average tem- 

 perature in the common frog. That animal cannot 

 bear a degree of heat equal to the average tempera- 

 ture of health in the human body ; and yet it passes 

 into a state of inactivity as soon as the cold of winter 

 begins to set in. In the boas, and it is the same 

 with many other creatures which inhabit similar 

 places, we have instances of constitutions tempered to 

 the very hottest climates only, but which are inca- 

 pable of bearing the greatest heat even there. Indeed 

 incapacity of enduring the direct heat of the sun 

 when it beats in the intensity of its fervour upon the 

 dry and parched surface, appears to be one of the 

 reasons which confines these creatures to the thickets, 

 the marshes, the banks of the rivers, and, when the 

 sun is hot, and the marsh nearly dry, to the water 

 itself. Nor if we examine all nature shall we find any 

 instance in which the ranges upward and downward 

 are not in proportion to each other. That they shall 

 show the same range according to the scale of the 

 common thermometer, as for instance that an ani- 

 nml which is in its most natural activity at H0, and 

 becomes still at 3*2, shall become as dull at 90, is 

 another matter, and one which we cannot expect. The 

 degrees of the thermometer, are merely equal expan- 

 sions of the mercury, or spirit of wine, or other ther- 

 mometric substance ; and we know nothing of the 

 relative powers of heat by which these are effected, 

 only we trace the most apposite analogies in favour of 

 the hypothesis that the force of heat requisite for pro- 

 ducing equal expansions not only varies with every 



