SIMPLE AND COMPOUND EVOLUTION. 313 



generally, the amounts of structure and rates of structural 

 change, are smaller throughout the vegetal kingdom than 

 throughout the animal kingdom; and, speaking generally, 

 the heat of plants is less than the heat of animals. A com- 

 parison of the several divisions of the animal kingdom with 

 one another, discloses among them parallel relations. Re- 

 garded as a whole, vertebrate animals are higher in tempera- 

 ture than invertebrate ones ; and they are as a whole higher 

 in organic activity and complexity. Between subdivisions 

 of the vertebrata themselves, like differences in the state of 

 molecular vibration, accompany like differences in the de- 

 gree of evolution. The least compounded of the Vertebrata 

 are the fishes ; and in most cases, the heat of fishes is nearly 

 the same as that of the water in which they swim: only 

 some of them being decidedly warmer. Though we habit- 

 ually speak of reptiles as cold-blooded; and though they 

 have not much more power than fishes of maintaining a tem- 

 perature above that of their medium ; yet since their medium 

 (which is, in the majority of cases, the air of warm climates) 

 is on the average warmer than the medium inhabited by 

 fishes, the temperature of the class of reptiles is higher than 

 that of the class of fishes ; and we see in them a correspond- 

 ingly higher complexity. The much more active molecular 

 agitation in mammals and birds, is associated with a consid- 

 erably greater multiformity of structure and a very far 

 greater vivacity. The most instructive contrasts, 



however, are those occurring in the same organic aggregates 

 at different temperatures. Plants exhibit structural changes 

 that vary in rate as the temperature varies. Though light 

 is the agent which effects those molecular changes causing 

 vegetal growth, yet we see that in the absence of heat, such 

 changes are not effected: in winter there is enough light, 

 but the heat being insufficient, plant-life is suspended. 

 That this is the sole cause of the suspension, is proved by 

 the fact that at the same season, plants contained in hot- 

 houses, where they receive even a smaller amount of light, 

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