Chap. XI.J OF ORGANIC BEINGS. 291 



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 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 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 simultaneously, or to an equal degree. Thc- 

 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 irnrnigration 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 identical 

 form much longer than others ; or, if changing, should change in a 

 less degree. We find similar relations between the existing inha- 

 bitants of distinct countries; for instance, the land-shells and 

 coleopterous 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 organised productions compared with marine 

 and lower productions, by the more complex relations of the higher 

 beings to their organic and inorganic conditions of life, as explained 

 in a former chapter. When many of the inhabitants of any area 

 have become modified and improved, we can understand, on the 

 principle of competition, and from the all-important relations of 



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