296 TJIE GEOLOGICAL SUCCESSION Chap. X. 



livinfj: spocics of this p^'onus ; whereas most of the other Silu- 

 rian Molhiscs and all the; Crustaceans have changed greatly. 

 The productions of the land seem to change at a quicker rate 

 than those of the sea, of whicli a striking instance has lately 

 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 JM. Barrande, which 

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

 then allow the preexisting fauna to reappear ; but Lyell's ex- 

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

 from a distinct geographical province, seems to me satisfac- 

 tory. 



These several facts accord well Avith our theory, which in- 

 cludes no fixed law of development, causing all the inhabitants 

 of an area to change abruptl}^, or simultaneously, or to an equal 

 degree. Tlic 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 quite 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 con- 

 tingencies — 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 

 means surprising that one species should retain the' same iden- 

 tical form much longer than others ; or, if changing, that it 

 should chang(! in a less degree. We find similar relations be- 

 tween the inhaljitants of distinct countries ; for instance, the 

 land-sliclls and coleopterous insects of jNIadeira have come to 

 diller considerably from their nearest allies oh the continent 

 of l^^urope, whereas the marine shells and birtls have remained 

 unaltered. We can perhaps understand the apparently quicker 



