166 DR H. B. FAXTHAM 



at the end of seven 3'ears (a short life, excluding violent 

 deaths, for any bird) there will be 2048 birds, instead 

 of the original sixteen. As this increase is quite im- 

 possible, we must conclude either that birds do not rear 

 nearly half their young, or that the average life of a bird 

 is, from accident, not nearly seven years. Both checks 

 probably occur. The same kind of calculation applied to 

 all plants and animals affords results more or less strik- 

 ing, but in very few instances more striking than in man." 



"In nature we have some slight variation occasionally 

 in all parts; and I think it can be shown that changed 

 conditions of existence is the main cause of the child not 

 exactly resembling its parents; and in nature geology 

 shows us what changes have taken place, and are taking 

 place. We have almost unlimited time." 



However, the increase of living animals depends on 

 very many and diverse conditions. Thus, Darwin dis- 

 cusses the case of a barren heath planted with Scotch 

 fir. The change in the native vegetation of the planted 

 part of the heath was most remarkable, greater than that 

 generally seen in passing from one soil to a quite dif- 

 ferent one. Twelve species of plants flourished in the 

 plantations which could not be found on the heath. The 

 effect on the insects must have been still greater, for six 

 insectivorous birds were very common in the plantation, 

 which were not found on the heath. 



Again, fertilisation of certain plants can only be 

 effected by definite insects. Thus, the visits of bees are 

 necessary for the fertilisation of clover. The reproduction 

 of red clover, however, is dependent on humble bees. The 

 , number of humble bees depends in a great measure upon 

 the number of field-mice which destroy their nests, and 

 the number of mice is dependent on the number of cats. 

 Thus the number of cats in a district might determine 

 tlifle frequency of red clover in that district. 



Fn 1858 he wrote: "Another principle, which may be 

 called the principle of divergence, plays, I believe, an 



