340 MR. R. I. POCOCK ON THE [ Apr. 21, 
Now, under the conditions of mammalian life—conditions in which 
survival of the fittest attains within its limited sphere of operation 
a maximum degree of efficiency—it is obvious that diminution of 
activity would be fatal, the speed of mammals being one of the 
most important conditions to survival in the struggle for existence 
(as is shown by the fact that this trait is so highly developed in 
these animals) ; hence, a less degree of activity bemg prohibited, 
any decrease in the mass of the ovary would be of service to the 
organism in which it occurred. Natural selection may legitimately 
be supposed to operate here, since, although it has been clearly 
demonstrated that the higher the life of the organism the less 
range of application does this principle possess, yet it doubtless 
applies in the case of any feature which is of paramount importance, 
and modifications concerned with the genital structures must 
necessarily possess such importance. It is therefore possible, and 
even probable, that the loss of yolk suffered by the mammalian 
ovary and the alternative adoption of a placental mode of nutrition 
both indirectly result from that same cause of impulsive locomotion 
to which we have traced several other features of mammalian 
structure. 
It may also be worth while to add that many minor features of 
mammalian anatomy, the significance of which is usually over- 
looked, are only explicable on the assumption that they are related 
to impulsive locomotion. Instances of these minor structures are : 
the accumulation of fat at the base of the heart, the fatty cushion 
surrounding the neck of the bladder, the fatty development about 
the kidney already noticed, and the various ‘“ suspensory ligaments ” 
and other “ fixative organs” referred to above, associated with the 
stomach, liver, and other viscera of large mass. 
d. On the Geographical Distribution of Spiders of the 
Order Mygalomorphe. By R. I. Pococxr, F.Z.S. 
[ Received March 17, 1903.] 
(Text-figures 58-61.) 
Parr [. 
InrRODUCTORY REMARKS UPON THE PALHZONTOLOGY AND THE 
MEANS OF DISPERSAL OF SPIDERS. 
(a) Summary of the Paleontological History of Spiders, and 
its bearing on the Phenomena of Distribution. 
Owing to the enormous chances against the preservation of 
fossil Spiders in sedimentary rocks, the paleontological history 
of this Order is very imperfect. One or two types have been 
diseovered in Carboniferous strata of Europe and North America 
(Arthrolycosa and Protolycosa), and also a fairly large number of 
specimens from amber and from gypsum and lacustrine deposits 
of Oligocene and Miocene age in those countries. But absolutely 
