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Show 335 MR. O. THOMAS ON THE RACES OF ECHIDNA. [Apr. 21, other, and that a male lawesi would probably be very much more like aculeata than are the two females examined. To show this the dimensions of a hypothetical male skull, based by simple rule of three on the relations to each other of specimens r and s', the typical male and female skulls of setosa already described and figured, have been placed below those of the female specimens of lawesi in the table, and these, which are probably not very far from what average male skulls would measure, show that no sharp dividing line can be drawn between the skull proportions of lawesi and aculeata. Still less can one be drawn between aculeata and setosa, as the various numbers intergrade completely. I regret that I am unable to retain, even as a variety, E. acanthion, Collett, the types of which have been kindly lent to me by the describer. It seems to me to be what I might call a hyper-typical form of aculeata, not worthy of a separate name, but exceedingly interesting as supplying the much needed intermediate link between E. aculeata and E. lawesi. With regard to the interesting character of cranial capacity, the variation between the different races and individuals is extremely striking, such a range as from 17 to 37 c. cm. being probably unequalled among mammals ; and even within the varieties we find such ranges as from 22 to 27 in aculeata, and from 27 to 37 in setosa. The even increase of capacity, however, from north to south is a fact of great interest, and gives an excellent example of the general law as to increase of size with increase of latitude, which is now one of the most fully recognized of the laws governing the variation of mammals. This law, however, as Mr. J. A. Allen has shown \ is reversed in the case of essentially tropical groups, their members then becoming smaller and smaller according as they live further and further away from what Mr. Allen calls their " centre of distribution." The fact therefore that Echidna conforms to the general rule is exceedingly interesting, and tends to prove that it is essentially a temperate and not a tropical genus, and that the New-Guinean E. lawesi must be looked upon as a more or less degenerate tropical offshoot of E. aculeata. But, on the other hand, speaking of the whole family, its very largest member, the Proechidna bruijnii, occurs at the most northern and tropical situation of all, namely in north-western New Guinea; so that this is in direct contradiction to Mr. Allen's further rule that the largest species of a family are those that have their habitat nearest to its "centre of distribution." W e have therefore in the Echidnida the apparent anomaly of two centres of highest development, the one, tropical, applying to the family as a whole, and causing P. bruijnii to be its largest member, and the other, temperate, applying to the individuals of the only widely spread species, and causing them to increase steadily in size from north to south. On the whole I think that the facts as to the relations to each other of distribution and size in this group tend to show that the genus Echidna has existed more or less in its present form for a Bull. TJ. S. Geol. Surv. ii. p. 310. |