THE HAUNTS OF LIFE 119 



secondary return to the water is illustrated by some Reptiles 

 — water-snakes, turtles, crocodilians and a single marine 

 lizard (Amblyrhynchus) ; by some birds like the flightless 

 penguins and the pelagic petrels ; by some mammals like 

 Cetaceans and Sirenians, seals and sea-lions. 



Origin. — Some terrestrial animals probably passed 

 from the freshwaters, through the mediation of marsh 

 and bog. The earthworms form a large cosmopolitan 

 group, now thoroughly terrestrial and indeed avoiding 

 very wet places, but the occurrence of three or four aberrant 

 types (like Alma and Dero) with gills tells the tale of their 

 historical origin. No one can doubt that the land-leeches 

 were derived from a freshwater stock, for the great majority 

 of leeches (Hirudinea) are tenants of the freshwaters. It 

 is probable that the snails and slugs of dry land originated 

 from a freshwater stock and there is, of course, no dubiety 

 in cases like frogs and toads where the larval life is still 

 spent in the ponds and ditches. The interesting land-crab, 

 Birgus latro, which goes far up the mountains and even 

 cUmbs trees, returns every year to the sea-shore to breed, 

 and its marine larvae well illustrate the general conclusion 

 that the habitat of the yoimg forms is the ancestral habitat. 

 It is possible that the terrestrial Isopods were also derived 

 from a littoral stock. 



If a land-animal has not originated from a freshwater 

 stock or from a littoral stock, how else could it arise ? 

 The third mode of origin is from some pre-existing terres- 

 trial stock. Thus Mammals probably evolved from a 

 terrestrial Reptile stock, and Reptiles from a terrestrial 

 Amphibian stock. Thus, again, it is probable that Insects 

 and Spiders sprang from pre-existing terrestrial stocks of 

 Arthropods. 



