282 ON THE ORIGIN OF SPECIES 



inhabitants of a country to change abruptly, or simul- 

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

 fication must be extremely slow. The variability of 

 each species is quite independent of that of all others. 

 Whether such variability be taken advantage of by 

 natural selection, and whether the variations be accu- 

 mulated to a greater or lesser amount, thus causing a 

 greater or lesser amount of modification in the varying 

 species, depends on many complex contingencies, — on 

 the variability being of a beneficial nature, on the 

 power of intercrossing, on the rate of breeding, on the 

 slowly changing physical conditions of the country, 

 and more especially on the nature of the other 

 inhabitants with which the varying species comes 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, that it should 

 change less. We see the same fact in geographical 

 distribution ; for instance, in the land-shells and 

 coleopterous insects of Madeira having 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 in- 

 organic conditions of life, as explained in a former 

 chapter. When many of the inhabitants of a country 

 have become modified and improved, we can under- 

 stand, on the principle of competition, and on that of the 

 many all -important relations of organism to organism, 

 that any form which does not become in some degree 

 modified and improved, will be liable to be exter- 

 minated. Hence we can see why all the species in 

 the same region do at last, if we look to wide enough 

 intervals of time, become modified ; for those which do 

 not change will become extinct. 



In members of the same class the average amount of 

 change, during long and equal periods of time, may, 



