42 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



partially adjusted to an air-breathing life. A few of these have led to 

 the establishment of new and successful groups of terrestrial animals, 

 but for the most part the adaptation to air-breathing came to an 

 early stop, without occasioning the complete transformation of the 

 mode of life. Attempts to enter the terrestrial habitat which have met 

 with a limited or partial success are usually confined to isolated 

 genera or to small groups. Invertebrate groups of this kind are the 

 Onychophora, many isopods and amphipods {Orchestia) , the ter- 

 restrial hermit crabs, land leeches, and land planarians. Among ver- 

 tebrates a number of groups of fishes from various families and orders 

 illustrate this tendency, as for example Misgurnus (Cobitidae), Sac- 

 cobranchus (Siluridae), the eel-like Amphipnous, the climbing-perch 

 Anabas, Periophthalmus, and all the lungfishes. The apparatus for 

 air-breathing among these fishes is diverse. That shown by the lung- 

 fishes, consisting of a pair of diverticula of the anterior part of the 

 alimentary canal, has proved successful; some such mechanism made 

 possible the development of the air-breathing vertebrates from the 

 parent stock of crossopterygian fishes. The invertebrate groups which 

 have solved the problem of air-breathing, and have undergone a re- 

 newed evolutionary development in consequence, are the pulmonate 

 snails, among the mollusks, and the myriapods, insects, and arachnids 

 among the arthropods. 



In consequence of its low density, the air offers much less resist- 

 ance to motion than water, and the presence of the solid earth as a 

 basis also favors more rapid motion than is possible for aquatic ani- 

 mals. In spite of the low density of the air, a number of groups of 

 animals have independently acquired the power of flight. In view of 

 the unbroken extent of the atmosphere, flight is the most perfect form 

 of locomotion. It has been mastered by four groups, the insects, the 

 reptiles (in extinct forms), the mammals (bats), and the birds. 



Another advantage gained by the adoption of the terrestrial mode 

 of life was the enormous amount of previously unavailable plant food. 

 Land plants, which attained a high development before the appearance 

 of the terrestrial animals, afforded a food supply for which there were 

 at first only a few competitors. The number of herbivores among in- 

 sects and myriapods, which may perhaps be regarded as the earliest 

 terrestrial animals, is still large. The herbivorous insects include the 

 cockroaches and grasshoppers, the termites, the earwigs, many beetles 

 and Hymenoptera, many Diptera, and most Lepidoptera. The pul- 

 monate snails, also, are almost exclusively herbivorous. The success 

 of the herbivores in the terrestrial habitat made possible the entrance 

 of carnivores, such as the arachnids and vertebrates, and favored the 



