THE DISPERSAL OF SPIDERS 

177

III. THE DISPERSAL OF SPIDERS. 

 In the year that L. Paulus and C. Marcellus were consuls, it rained wool.    

Pliny

. 

For several years L. Berland and I have been in disagreement as to 

the relative importance of the different factors which influence 

distribution. The friendly nature of this contest has been such that 

should either of us in the future be converted to the other's cause there 

would remain, I feel certain, nothing but pleasure for both of us at the 

realization that our joint efforts had ultimately brought us to the same 

conclusions. 

Berland believes that the distribution of species today can be 

explained primarily in terms of (1) former land connections and (2) 

the unwitting transportation of spiders by man in his cargo and ships. 

My views, on the other hand, are (1) that many of the assumed land 

bridges vanished, if they ever existed, too long ago to serve as an 

explanation of the present distribution of species, (2) that man has 

undoubtedly provided the agency by which many species have been 

introduced from one country to another, and (3) that a more important 

factor is the constant interchange of species between neighbouring 

lands by air. 

A. LAND BRIDGES. 

Out of the hundreds of species which have been found embedded in 

Oligocene amber not one, it seems, has survived till the present day, 

so it follows that if importance is to be placed on land bridges these 

bridges must have been in existence at least until Miocene times. In 

point of fact the changes in climate which have occurred since 

Miocene times over a large part of the globe make it