344 TEE OEOLOQIGAL SUCCESSION 



Falconer has given a striking instance of a similar fact, 

 for an existing crocodile is associated with many lost 

 mammals and reptiles in the snb-Himalayaii deposits. 

 The Silurian Lingula differs but little from the living 

 species of this genus; whereas most of the other Silurian 

 Molluscs and all the Crustaceans have changed greatty. 

 The productions of the land soem to have changed at a 

 quicker rate than those of the sea, of which a striking 

 instance has been observed in Switzerland. There is some 

 reason to believe that organisms high in the scale, change 

 more quickly than those that are low: though there are 

 exceptions to this rule. The amount of organic change, 

 as Pictet has remarked, is not the same in each successive 

 so-called formation. Yet if we compare any but the most 

 closely related formations, all the species will be found to 

 have undergone some change. When a species has once 

 disappeared from the face of the earth, we have no reason 

 to believe that the same identical form ever reappears. 

 The strongest apparent exception to this latter rule is that 

 of the so-called ** colonies" of M. Barrande, which intrude 

 for a period in the midst of an older formation, and then 

 allow the pre-existing fauna to reappear; but Lyell's expla- 

 nation, namely, that it is a case of temporary migration 

 from a distinct geographical province, seems satisfactory. 

 These several facts accord well with our theory, which 

 includes no fixed law of development, causing all the 

 inhabitants of an area to change abruptly, or simultane- 

 ously, or to an equal degree. The process of modification 

 must be slow, and will generally affect only a few species 

 at the same time; for the variability of each species is 

 independent of that of all others. Whether such varia- 

 tions or individual differences as may arise will be aocu- 

 mulated through natural selection in a greater or less 

 degree, thus causing a greater or less amount of perma- 

 nent modification, will depend on many complex contin- 

 gencies — on the variations being of a beneficial nature, on. 

 the freedom of intercrossing, on the slowly changing 

 physical conditions of the country, on the immigration of 

 new colonists, and on the nature of the other inhabitants 

 with which the varying species come into competition. 

 Hence it is by no menus surprising tliat one species should 

 retain the same identical form much longer than others; 



