498 DR. A. W. ROWB ON THE GENUS MICKASTEE. [Aug. 1 899, 



the horizon, and not the species, which rules the issue ; the species 

 are but culminating points of certain dominant horizonal characters. 



A convincing proof of this contention is found in the upper part of 

 the Micraster cor-anguinum-zone, where broad forms — contrary to all 

 one's preconceived ideas — are equally as common as narrow forms ; 

 and yet a careful analysis of every broad form fails to furnish a single 

 example which can be called M. cor-testudinarium. The conditions 

 of existence were so powerful that M. cor-testudinarium could not 

 persist as such in the higher zones, and it became merely a broad 

 variety of M. cor-anguinum auctorum, with high-zonal characters, 

 just as M. cor-testudinarium, Goldfuss, in the low zones, is only a 

 broad variety of M. precursor. This is but another instance of the 

 unity of the scheme of evolution in this genus, for we find broad 

 varieties of narrow forms in every horizon, from the lowest to the 

 highest. 



We have persisted for so many years, adding species to species, 

 till many of the accentuated forms have now been described, and the 

 synonym}^ has already reached burdensome proportions. Unfortu- 

 nately, so prone is this plastic genus to variation that the material 

 for elevating mutations into species is by no means exhausted, and 

 one is compelled to ask whether the time has not come to call 

 a halt, to examine our position, and to consider whether there is 

 no way out of a situation which is rapidly becoming untenable. 

 Two courses are open to us. We must either follow the Continenta 

 school, and make every prominent form a species, on the plea that 

 every minute variation must be ticketed and pigeonholed, irrespective 

 of the fact that such variations may be valueless as zonal guides ; 

 or we must strike out in the new direction to which English thought 

 18 certainly tending, and study the facies of the genus as a whole, 

 carving out broad zoological groups, and allowing the horizon, and 

 not the species, to be our criterion. In this way, all valid species 

 and varieties will be retained, and those which are valueless as 

 2onal guides will soon find their level, and sink into the oblivion of 

 an unwieldy synonymy. 



To such a scheme as this the writer unhesitatingly gives his 

 allegiance, for in it we may hope to trace the evolution of the genus 

 as a whole, and each prominent feature of the test in particular; and 

 in it the neglected, but equally valuable passage-form, will receive 

 its due recognition. 



Unless a genus like Micraster is attacked from the zoological 

 standpoint, all trace of continuity and evolution is lost, and the 

 whole enquiry fritters away into an attempt to learn by heart the 

 names and shapes of certain marked forms, disconnected, and with- 

 out meaning. The clue to the whole question lies in the narrow zone 

 of Holaster planus, for if we can unravel the tangle in this zone all 

 the rest becomes clear. In this zone we find M. cor-hovis, M. Leskei^ 

 M. prcBCursor, M. cor-testudinarium, and but for the interporiferous 

 area of the paired ambulacra it would be almost impossible to 

 distinguish the latter two from the same forms in the zone imme- 

 diately above. 



