330 MR. O. THOMAS ON THE RACKS OF ECHIDNA. [Apr. 21, 



Other, and that a male Jawesi 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. acanf/iion, 

 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 hvper-typical form of 

 aculeata, not worthy of a separate name, but exceedingly interesting 

 as supplying the much needed intermediate link between^, aculeata 

 and JE. 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 1/ to 37 com. being probably un- 

 equalled 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-Guiiiean E. lawesi must be 

 looked upon as a more or less degenerate tropical offshoot of £". acu- 

 leata. But, on the other hand, speaking of the whole famil)', its 

 very largest member, the Proechirlna 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." We have therefore 

 in the Echidmdce 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, tempe- 

 rate, applying to the indiA'iduals 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 



1 Bull. U. S. Geol. Surv. ii. p. 310. 



