DISTRIBUTION BY ENVIRONMENTS 

103

II. DISTRIBUTION BY ENVIRONMENTS 

The nature of the spider fauna of different countries varies 

considerably. It we take the number of species belonging to each 

family as providing us with some rough idea of dominance after 

calculating the number in each case as a percentage of the total spider 

fauna, we find (

a

) that some, such as the Mygalomorphs, Salticids and 

Sparassids, flourish in the tropics and diminish progressively as we 

pass into cooler regions, (

b

) that others, including the Drassidae, 

Dysderidae, Agelenidae, Zodariidae and the Cribellate families, thrive 

most successfully in the warmer temperate or sub tropical areas, and 

(

c

) that others, including the Lycosidae and particularly the 

Linyphiidae, reach their greatest dominance in the Arctic or sub 

Arctic regions. An analysis of the known faunas of a representative 

selection of countries is given below in percentages (see p. 104). 

It cannot be doubted that the main cause of these differences in the 

constitution of the faunas is climatic. Even when we take such a small 

area as that covered by the British Isles we find marked differences 

between the north and the south, differences which demonstrate the 

same trends as those contained in the table given above. Scotland 

lacks Mygalomorphs, Oxyopids, Pholcids, Scytodids, Sparassids, 

Eresids and Uloborids, whilst the Linyphiids and Salticids represent 

50% and 4% of its fauna respectively, compared with 42% and 6% for 

England. 

Though I do not regard the average annual temperature of an area as 

being in itself a deciding factor in the potential distribution of a 

species, it is worthy of note