Other groups of facts prove that the quantity of secondary re-distributionin an organism varies, caeteris paribus, according to the contained quantityof the motion called heat. The contrasts between different organisms, anddifferent states of the same organism, unite in showing this. Speaking generally,the amounts of structure and rates of structural change, are smaller throughoutthe vegetal kingdom than throughout the animal kingdom; and, speaking generally,the heat of plants is less than the heat of animals. Comparisons of the severaldivisions of the animal kingdom with one another, disclose parallel relations.
Regarded as a whole, vertebrates are higher in temperature than invertebrates;and they are as a whole higher in activity and complexity. Between subdivisionsof the Vertebrata themselves, like differences in the degrees of molecularvibration accompany like differences in the degrees of evolution. The leastcompounded of the Vertebrata are the fishes; and, usually, the heat of fishesis nearly the same as that of the water in which they swim: only some largeones being decidedly warmer. Though we habitually speak of reptiles as cold-blooded,and though they have not much more power than fishes of maintaining a temperatureabove that of their medium, yet since their medium (which is, in the majorityof cases, the air of warm climates) is on the average warmer than the mediuminhabited by fishes, the temperature of the class reptiles is higher thanthat of the class fishes; and we see in them a correspondingly higher complexity.
The much more active molecular agitation in mammals and birds, goes alongwith a considerably greater multiformity of structure and a far greater vivacity.
The most instructive contrasts, however, are those occurring in the sameorganic aggregates at different temperatures. Structural changes in plantsvary in rate as the temperature varies. Though light effects those molecularchanges causing vegetal growth, yet in the absence of heat, such changesare not effected: in winter there is enough light, but not enough heat. Thatthis is the sole cause of the suspension of growth, is proved by the factthat at the same season, plants contained in hot-houses go on producing leavesand flowers. We see, too, that their seeds, to which light is not simplyneedless but detrimental, germinate only when the return of a warm seasonraises the rate of molecular agitation. In like manner the ova of animals,undergoing those changes which produce structure in them, must be kept moreor less warm: in the absence of a certain amount of motion among their molecules,the re-arrangement of parts does not go on. Hybernating animals also supplyproof that loss of heat carried far, retards extremely the vital transformations.
In animals which do not hybernate, as in man, prolonged exposure to intensecold causes extreme sleepiness, which implies a lowered rate of organic changes;and if the loss of heat continues, there comes death, or stoppage of thesechanges.
