PRELIMINARY CONSIDERATIONS. 59 



all events an approach to within a very few miles 

 of each other, has at one time existed." Besides 

 the groups referred to, I claim that particular 

 attention should be devoted to Amphibia, which, 

 contrary to Wallace, I hold do not possess special 

 facilities for dispersal ; and also to spiders and to 

 all wingless animals leading a subterranean life, such 

 as some of the wood-lice, planarian worms and 

 apterous beetles. 



A thorough knowledge of the changes in the dis- 

 tribution of land and water is desirable in order to 

 appreciate the extent and variations of former mi- 

 grations. A study of the British fauna, for example, 

 teaches us that the British Islands were once con- 

 nected with one another and with the continent of 

 Europe between England and France. It was 

 Professor James Geikie, I believe, who first pointed 

 out, many years ago, that the area now covered by 

 the Irish Sea was formerly in all probability a fresh- 

 water lake. This had its outlet at the southern ex- 

 tremity in the form of a stream into which most likely 

 flowed the smaller rivers from the south-east of 

 Ireland, and which was joined from the east by the 

 Severn, and finally debouched into the Atlantic 

 (Fig. 4). The range in the British Islands of those 

 species which have migrated to them from the south, 

 indicates that whilst the Atlantic Ocean had gradually 

 crept up and flooded the area between Ireland and 

 Wales, and had turned the fresh-water lake into a bay, 

 communication between Scotland and Ireland was 



