DARWINISM AND EVOLUTION. II 



famine. Warlike foes burst into peaceful lands. Conquest para- 

 lyses commerce, and inventions revolutionise trade. 



7. Every group of organisms must accordingly change in 

 average character, under penalty of extinction. An organism not 

 in equilibrium with its environment must perish. 



The crust of the earth is crowded with the remains of extinct 

 plants and animals. Species, and genera, and orders ; all 

 extinguished, and by what ? The mammoth and the rhinoceros, 

 the sabre-toothed tiger and the lion that in former days haunted 

 the valley of the Thames, why have they left no descendants ? 

 From failure of food, from change of climate, from mutual 

 slaughter, from the hand of man ? Be the cause what it may, 

 they failed to adapt themselves to an altered environment, and 

 they perished. 



Some existing marine animals, like the lingula and the 

 terebratula, of lowly types, seem to have varied very little even 

 during an enormous lapse of time. As the fossils which represent 

 the ancestors of these molluscs consist only of their shell-forms, 

 as the animals themselves have in all instances absolutely vanished, 

 and as the shell is but a feeble index to the structure of its con- 

 tents, we can only infer from analogy that some degree of 

 evolution has taken place. 



All that the proposition states is that organisms must be in 

 equilibrium with their environment. If the environment is 

 constant, so must be the organism, when an equilibrium has been 

 once attained. Now, marine changes are chiefly those of tem- 

 perature and depth, and as these vary with extreme slowness, all 

 that would be necessary on the part of the bivalves in question, 

 in order to adjust themselves to external conditions, would be to 

 shift their position, a little to the north or a little to the south, as 

 their case might require.* There is one structural adjustment, 

 however, which even molluscs must make on pain of extinction. 



Water, marine or fluviatile, contains in solution a varying 

 quantity of carbonic acid, which has a destructive action on those 

 shells that are made of carbonate of lime. 



Hence the shell of molluscs is covered with a layer of animal 

 matter, called an epidermis. It is of a horny consistence, and, 

 serving to protect the shell from the action of carbonic acid, is of 

 greater or less thickness according to the quantity of acid dissolved 

 in the surrounding medium. In fresh water this quantity is often 

 very large, and it is on fresh-water shells that the epidermis attains 

 its greatest thickness. In some localities, where the animal has 

 failed to adjust itself to the environment, and the epidermis is 

 insufficiently thick, the substance of the shells is found deeply 

 corroded, t 



* The existing Lingulse are found only in southern seas ; the existing 

 Terebratulee only in very deep water. 



t The valves of the existing Lingulte are of a very horny texture. 



