XXVlll INTRODUCTION. 



belief in an Almighty Creator and a Divine Wisdom, who 

 has created the heavens and the earth according to an 

 eternal and preconceived plan, can alone resolve the 

 enigmas of nature, as well as those of human life. Let 

 us still erect statues to men who have been useful to 

 their fellow-creatures, tind have distinguished themselves 

 by their genius, but let us not forget what we owe to Him 

 who has placed marvels in each grain of sand, a world 

 in every drop of water." 



At first we shall treat of animal messmates, secondly 

 of mutualists, and thirdly of parasites. 



