io NATURAL HISTORY OF AQUATIC INSECTS 



lie within a ring-fence, and the facts of distribution 

 will soon tell us whether a group is dominant or not. 

 It is especially characteristic of the most dominant 

 groups that they can hold their own in the wider 

 areas, where the struggle for existence is most severe. 

 Groups which can maintain themselves in the wide 

 continents of the northern hemisphere, will easily 

 prevail over the populations of the southern islands ; 

 groups which can maintain themselves in the vast 

 fresh-water lakes of North America, will easily occupy 

 the small fresh-water lakes and ponds of England, if 

 ever they are brought into competition with the in- 

 digenous population. 



All the various marks of dominance are united in 

 Insects, and are there exhibited in a striking degree- 

 Numerical abundance, zoological continuity, or the 

 co-existence of many allied forms, geographical con- 

 tinuity, and predominance in the widest areas of 

 competition distinguish the whole class. There are, 

 of course, degrees of dominance among them. The 

 Diptera are eminently dominant among Insects, the 

 Muscidae dominant among Diptera, and so on. The 

 extraordinary variety in form, size, and habitat which 

 Insects present is in keeping with all that we know 

 of their range and frequency. 



What are the qualities which confer dominance 

 upon Insects or any other group of animals ? The 

 question is too hard for us. It may be one quality 

 here, and another there. But though we must not 

 speak with confidence on so difficult a question, there 

 is some likelihood in the supposition that adaptability 

 to new conditions would be one ground of success. If 



