INTRODUCTION. 23 



All zoologists, indeed, are quite in accord with Dr. 

 Wallace's view as expressed in Island Life (p. 74). 

 " Whenever we find that a considerable number of 

 the mammals of two countries exhibit distinct marks 

 of relationship, we may be sure that an actual land- 

 connection, or at all events an approach to within 

 a very few miles of each other, has at one time 

 existed." As all the European islands come under 

 this category, their mammals exhibiting distinct 

 relationship with those on the European continent, 

 they all have been connected with it formerly. 



Perhaps the most powerful of all agents in the 

 transportal of species by accidental means is man. 

 But his actions may be accidental as well as in- 

 tentional. We have therefore to distinguish between 

 the animals disseminated all over the world by pure 

 chance, and those which have been introduced into 

 new countries purposely. Invertebrates, such as 

 snails, centipedes, woodlice, beetles, and cockroaches, 

 are constantly being unintentionally carried with 

 vegetables, fruit, trees, and with timber from one 

 country to another. Earthworms are sometimes 

 transported in the balls of earth in which the roots 

 of trees are enveloped. As regards molluscs, Mr. 

 Kew believes (p. 178) that during the last three 

 centuries at least, human agency has influenced 

 their disposal more than all other causes taken 

 together. A large number of species of invertebrates 

 in America are said to owe their existence in that 

 country to accidental introduction by man. In 



