208 Journal of the Asiatic Society of Bengal. [June. 1914. 



tion at the centre of distribution results in conditions such as 

 are now found in the Indo-Australian Passalidae, a condition 

 which might well accelerate the advent of senescence in the 

 same area, with its accompaniment of "curious and eccentric 

 species" and the " great development of certain characters at 

 the expense of others, which usually leads to prompt extinc- 

 tion " — characters ' * which, so far as wo can see, serve no useful 

 purpose " (Amer. Nat. xlv, p. :*74). Some of the most highly 

 asymmetrical Passalidae might well bo termed curious and 

 eccentric ; nor is their asymmetry known to serve any useful 

 purpose 



The occurrence both among Passalids and among Crinoids, 

 to which the two hypotheses seem respectively to apply, of 

 single species having a geographical range coterminous with 

 those of all the species closely allied to it ; and tne occurrence 

 in the same groups of a well marked correlation, radial in 

 character, between distribution and specialization, suggests 

 that some connection between the two hypotheses is likely to 

 exist, in spite of apparent differences. 



Some of these differences are probably differences of inter- 

 pretation only ; for the two hypotheses have been worked out 

 quite independently. I had already noticed the radial dis- 

 tribution of the Thelyphonidae before the publication of Clark's 

 "Ontogeny of a Genus." And although this paper attracted 

 my attention at that time, my recollection of it lay dormant 

 throughout the whole period of my work on the Passalidae : 

 and it was only when searching for references in connection 

 with the preparation of the present paper that I recollected it, 

 and discovered, not only its important bearing on my work, but 

 also that o, its author's zoogeographical notes in " Crinoids of 

 the Indian Ocean." In view of the separate origins of our 

 respective hypotheses, and the many differences there must 

 have been in the facts noticed in connection with each, differ- 

 ences of opinion are almost certain to have arisen, and to have 

 resulted in our interpreting other facts, not essentially differ- 

 ent in themselves, from different points of view. 



Especially, t seems to me, have our points of view been 

 influenced by the difference in position in its group, of the single 

 variable and widely distributed species of each of the groups 

 with which we have respectively dealt. This difference is even 

 greater than appears at first sight from what has already been 

 said about it above. For although this species " typically falls 

 midway between the two extremes" in the instances from 

 which Clark's conclusions were drawn, it does so not because 

 it stands in the middle of the now existing sftction of a single 

 line of evolution in its group; but because it stands at the 

 base of two or more divergent lines and "is probably very 

 close to the original stock." Its genealogical position, con- 

 sequently is as widely removed as it could possibly be from the 



