GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 365 



given a striking instance of a similar fact, for an existing 

 crocodile is associated with many lost mammals and reptiles 

 in the suh-Himalayan 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 greatly. The productions of the land seem 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 dis- 

 appeared 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 explanation, 

 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 in- 

 habitants of an area to change abruptly, or simultaneously, 

 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 variations or individual 

 differences as may arise will be accumulated through natural 

 selection in a greater or less degree, thus causing a greater 

 or less amount of permanent modification, will depend on 

 many complex contingencies — 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 com- 

 petition. Hence it is by no means surprising that one species 

 should retain the same identical form much longer than 



