254 EVOLUTION AND GENETICS 



still retain the power of flight {Belostomatidae, Corixidae, Notonec- 

 tidae, Nepidae). These insects have the legs highly modified as 

 swimming organs. Other species (Gelastocoridae) are found along 

 shores, while still others (Veliidae, Gerridae) skate about on the 

 surface film of fresh waters. One genus of the Gerridae is marine. 

 This genus, Halobates, may be found far from land, and Beebe 

 has recently confirmed the belief that its eggs are deposited on 

 floating feathers. Some are fossorial (Gelastocoridae, Saldidae, 

 Cydninae). In food habits they vary greatly. Since they have 



Fossorial Aquatic-Volant 



Fossorial-Volant 



Water-striders 

 (Volarxt: 



Ambulalory-Volant 



' Stem Water-striders 



(Non-volant^ 



Ambulatory 

 (Non-volant) 



Fig. 151. — Diagram of adaptive branching in the Hemiptera. 



suctorial mouths, they are limited to liquid foods, but they include 

 both phytophagous and carnivorous forms. Some are highly 

 predacious, some parasitic, and some probably scavengers. 



Geographical Distribution. Animals that migrate, whether in 

 response to the pressure of necessity or to the inherent ability and 

 disposition to roam, have the world and its varying conditions 

 at their disposal. Within certain limits of time and space they 

 are restricted by the physical characteristics of the earth's surface, 

 but these features have changed repeatedly during the past, so 

 that what is now impossible may once have been easy. Migration 

 is likely to carry animals unchanged into favorable regions or to 

 carry them by gi-adual steps into regions which would demand 

 new adaptations. It has resulted in the population of the earth 

 with a unified fauna and flora. In similar habitats, however 

 remote, we may expect similar organisms, and only those land 

 masses whose isolation extends back beyond the origin of modern 

 species are peopled with organisms fundamentally different from 

 those of other similar regions. 



