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Show 340 MR. R. I. TOCOCK 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 being 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 overlooked, 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. 5. On the Geographical Distribution of Spiders of the Order Mygalomorphae. By R. I. P o c o c k , F.Z.S. [Received March 17, 1903.] (Text-figures 58-61.) P ar t I . I ntroductory R em ar k s upon th e P alaeontology a n d th e M eans of D isper sal of S p id e r s . (a) Summary of the Palaeontological 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 palaeontological history of this Order is very imperfect. One or two types have been discovered 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 |
