Mexican Amphibians and Reptiles. 711 
subjected to tropical conditions are mostly failures, since they ex- 
haust themselves through want of rest. Annuals seem naturally to 
have better chances than perennials. 
Temperate Lizards and Tortoises, if not allowed to hibernate, 
have their lives shortened, because they have lived “too much and 
too fast”, whilst tropical species of the same groups will do very 
well without such a rest. Of course many creatures aestivate, a 
condition in many respects resembling that of hibernation, but whilst 
the latter, if profound, implies almost complete suspension of meta- 
bolism and therefore little loss of substance, aestivation often ex- 
hausts them much, especially those which pass their torpor in dry 
surroundings. 
Cold can be counteracted in many ways, as by more food, 
motion, shelter, a more non-conductive coat etc, and if, as with 
most Amphibia and Reptiles, the temperature sinks niehtly or for 
a whole season, the body simply experiences a rest, to reawaken with 
the returning warmth. A hot-land species transferred to a temperate 
country, may find plenty of heat in the day time, even at night 
without much interruption during a whole season, sufficient for it 
to lead, so to speak, a three-quarter life each year, and tor all we 
know to the contrary, may thus prolong its entire life reckoned 
in years. 
Not so with a species which is transferred from the poikilo- 
thermous temperate to the hot zone with its much more equable 
climate, which implies always an excess of warmth over what that 
species was accustomed to. How the physiological process of the 
self adjusting regulation of the body’s temperature works in Reptiles, 
is unknown, as they neither possess sweatglands nor pant like some 
mammals. Personally I only know that their mysterious mechanism 
works most effectively. A snake or a lizard, when caught basking 
in the sun never feels hotter to the touch than the general tempe- 
rature, moreoften several degrees lower, but a short time after being 
left dead in the sun, upon the same spot where it was basking, it 
becomes disagreably hot. Black tortoises soon become as into- 
lerably hot, through and through, as a block of basalt. This different 
behaviour of the live and the dead body appears still more striking 
in our own climate, when, on a hot sunny summer’s day the live 
black tortoise, say a Cinosternum never gets overheated side by side 
with the dead body of another specimen. 
Let us apply the general principle, enunciated above, to geo- 
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