CHAP. XI.J OF ORGANIC BEINGS. 273 



Pictet has remarked, is not the same in each successive so-called 

 formation. Yet if we compare any but the most closely related 

 format ions, 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. 

 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 accumu- 

 lated 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 competi- 

 tion. 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 in- 

 stance, the land-shells and coleopterous insects of Madeira have 

 come to differ considerably from their nearest allies on the con- 

 tinent 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 organism to organism in 

 the struggle for life, that any form which did not become in some 

 degree modified and improved, would be liable to extermination. 

 Hence we see why all the species in the same region do at last, if 

 we look to long enough intervals of time, become modified, for 

 otherwise they would become extinct. 



In members of the same class the average amount of change, 



