Chap. XI.] OF ORGANIC BEINGS. 91 



includes no fixed law of development, causing all the 

 inhabitants of an area to change abruptly, or simul- 

 taneously, or to an equal degree. The process of modifi- 

 cation 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 

 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 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 under- 

 stand 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 in- 

 organic 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-import- 

 ant relations of organism to organism in the struggle for 

 26 



