Ofap. XIJ OF ORGANIC BEINGS. 91 



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 effect 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 sur- 

 prising that one species should retain the same identi- 

 cal form much longer than . others; or, if changing, 

 should change in a less degree. We find similar rela- 

 tions between the existing inhabitants of distinct coun- 

 tries; for instance, the land-shells and coleopterous in- 

 sects 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 organised 

 productions compared with marine and lower produc- 

 tions, 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 im- 

 proved, we can understand, on the principle of compe- 

 tition, and from the all-important relations of organ- 



