OF ORGANIC BEINGS. 328 



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 suc- 

 cessive 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 explanation, 

 namely, that it is a case of temporary migration from a dis- 

 tinct geographical province, seems satisfactory. 



These several facts accord well with our theory, which 

 includes no fixed law of development, causing all the inhabit- 

 ants 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 benefi- 

 cial nature, on the freedom of intercrossing, on the slowly 

 changing physical conditions of the country, on the immi- 

 gration 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 

 others ; or, if changing, should change in a less degree. We 

 find similar relations between the existing inhabitants of 

 distinct countries ; for instance, the land-shells and coleop- 

 terous insects of Madeira have come to differ considerably 

 from their nearest allies on the continent of Europe, whereas 

 the marine shells and birds have remained unaltered. We 

 can perhaps understand the apparently quicker rate of change 

 in terrestrial and in more highly organized productions com- 



