22 FOOT-NOTES TO EVOLUTION. 



ver, as other bees can not reach the nectar. . . . Hence 

 we may infer as highly probable that, if the whole genus 

 of humble-bees became extinct or very rare in England, 

 the heartease and red clover would become very rare 

 or wholly disappear. The number of humble-bees in 

 any district depends in a great measure on the number 

 of field mice, which destroy their combs and nests; and 

 Col. Newman, who has long attended to the habits of 

 humble-bees, believes that more than two thirds of them 

 are thus destroyed all over England. Now the number 

 of mice is largely dependent, as everyone knows, on the 

 number of cats ; and Col. Newman says, ' Near villages and 

 small towns I have found the nests of humble-bees more 

 numerous than elsewhere, which I attribute to the num- 

 ber of cats that destroy the mice.' Hence it is quite 

 credible that the presence of feline animals in large 

 numbers in a district might determine, through the in- 

 tervention first of mice and then of bees, the frequency 

 of certain flowers in that district." 



Huxley carries this calculation still further by show- 

 ing that the number of cats is dependent on the number 



of unmarried women. On the other 

 Relation of cats ^^^^^ ^j^^^^. pj-Qduces beef, and beef 

 to England s , ^, . , , 



Strength. 1 hus in a degree the prowess 



of England is related to the number of 

 spinsters in its rural districts. This statement would be 

 true in all seriousness were it not that so many other ele- 

 ments come into the calculation. But whether true or 

 not, it illustrates the way in which causes and effects in 

 biology become intertangled. 



The calculation has been lately made by Prof. Rufus 

 L. Green that at the normal rate of increase from a pair 

 of English sparrows, if none were to die except of old 

 age, it would take but twenty years to give one sparrow 

 to every square inch in the State of Indiana. But such 



