98 THE ORIGIN OF SPECIES 



it is a case of temporary migration from a distinct geo- 

 graphical province, seems satisfactory. 



Tliese 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 simul- 

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

 fication 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 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 exist- 

 ing 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 organized productions compared with 

 marine and lower productions, by the more complex rela- 

 tions of the higher beings to their organic and inorganic 

 conditions of life, as explained in a former chapter. 



