314 GEOLOGICAL SUCCESSION. Chap. X. 



These several facts accord well with my theory. I 

 believe in no fixed law of development, causing all the 

 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 ac- 

 cumulated to a greater or lesser amount, thus causing 

 a greater or lesser amount of modification in the vary- 

 ing 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 winch 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 Em-ope, 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 pro- 

 ductions compared with marine and lower productions, 

 by the more complex relations of the higher beingB 

 to their organic and inorganic conditions of life, as 

 explained in a former chapter. When many of the 

 inhabitants of a country have become modified and im- 

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

 petition, and on that of the many all-important rela- 

 tions of organism to organism, that any form which 

 does not become in some degree modified and improved, 



