172 Lieut. Spratt on the Influence of Temperature 



Atlantic ; and one species, the Littorina ccerulescens, extend- 

 ing from Tristan d'Acunha to the shores of Norway." It is 

 evident that the inhabitants of this region are subject to great 

 vicissitudes of temperature during the summer and winter 

 changes to which they are exposed. Nature having adapted 

 them to these conditions, we thus find that they are wanderers 

 through every clime; their geographic range corresponding 

 apparently to the local vicissitudes of temperature to which 

 they are subject. 



The lower subdivision of this region, and the region below 

 it reaching to the depth of 10 fathoms, is that in which the 

 vicissitudes of seasons is less felt; and that, consequently, in 

 which the average Mediterranean temperature has a more 

 permanent influence, the lowest part of the second region 

 being seldom at a lower temperature than 74-°at the sea-bottom. 

 It is in these two regions that Professor Forbes has shown 

 distinctly the influence of local temperature on the fauna ; for 

 he remarks " that it is in this belt we have the characteristic 

 species of the Mediterranean fauna." 



The third region extends to the depth of 20 fathoms, at 

 which the temperature has fallen to 68° ; but it has no very 

 characteristic fauna, Professor Forbes remarking that it may 

 be regarded as a zone of transition between the Mediterranean 

 fauna and that of the region below, where it presents well- 

 marked characters. 



The fourth region increases its range to 35 fathoms, at 

 which the temperature has decreased five or six degrees, being 

 generally 62° ; and, being so far beyond the influence of atmo- 

 spheric changes, is of a more permanent standard throughout 

 the year ; the consequence is, a more distinctly marked fauna, 

 and one upon which the influence of the decreased temperature 

 is decisive, for we find that the per-centage of Celtic forms in 

 this region is nearly 50 per cent. 



In the sixth region at the depth of 75 fathoms, the tem- 

 perature has nearly reached its minimum of 55° or 56°, a dif- 

 ference of only five or six degrees below that of the fourth 

 region. The Celtic forms are found rapidly to diminish in 

 the lowest region to 20 per cent., whilst in the sixth and seventh 

 regions it stands at 36 per cent. But Professor Forbes re- 

 marks, " that although we find fewer northern species in the 

 fauna of the lower zone, the number of forms representative 

 of northern species is so great, as to give them a much 

 more boreal or sub-boreal character than is present in those 

 regions where identical forms are more abundant." It must 

 be remembered, that our knowledge of the northern fauna is 

 not so perfect as that of the iEgean. This may partly be taken 



