312 SIMPLE AND COMPOUND EVOLUTION. 



Lepidosiren remains torpid in the hardened mud, until the 

 return of the rainy season brings water. Humboldt states 

 that during the summer drought, the alligators of the Pam- 

 pas lie buried in a state of suspended animation beneath the 

 parched surface, and struggle up out of the earth as soon as 

 it becomes humid. The history of each organism 



teaches us the same thing. The young plant, just putting 

 its head above the soil, is far more succulent than the adult 

 plant; and the amount of transformation going on in it is 

 relatively much greater. In that portion of an egg which 

 displays the formative processes during the early stages of 

 incubation, the changes of arrangement are more rapid than 

 those of which an equal portion of the body of a hatched 

 chick undergoes. As may be inferred from their respective 

 powers to acquire habits and aptitudes, the structural modifi- 

 ability of a child is greater than that of an adult man; and 

 the structural modifiability of an adult man is greater than 

 that of an old man : contrasts which are accompanied by cor- 

 responding contrasts in the densities of the tissues; since the 

 ratio of water to solid matter diminishes with advancing 

 age. And then we have this relation repeated in 



the contrasts between parts of the same organism. In a 

 tree, rapid structural changes go on at the ends of shoots, 

 where the ratio of water to solid matter is very great ; while 

 the changes are very slow in the dense and almost dry sub- 

 stance of the trunk. Similarly in animals, we have the con- 

 trast between the high rate of change going on in a soft 

 tissue like the brain, and the low rate of change going on 

 in dry non-vascular tissues, such as those which form hairs, 

 nails, horns, &c. 



Other groups of facts prove, in an equally unmistake- 

 able way, that the quantity of secondary re-distribution in 

 an organism varies; ccete ris paribus, according to the con- 

 tained quantity of the motion we call heat. The contrasts 

 between different organisms, and different states of the 

 same organism, unite in showing this. Speaking 



