GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 297 



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 forma- 

 tion. Yet if we compare any but the most closely related forma- 

 tions, 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 ex- 

 planation, 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 in- 

 cludes no fixed law of development, causing all the inhabitants of 

 an area to change abruptly, or simultaneously, or to an equal 

 degree. The process of modification must be slow, and will gen- ^' 

 erally 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 accu- 

 mulated 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 immi- 

 gration 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 inhabitants 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 organized 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 in- 



