612 



SCIENCE 



[N. S. Vol. XLII. No. 1087 



completed more tlian three years ago, but has 

 only recently been published.'^ However, little 

 has appeared in the interim which bears 

 directly on the subject and the author has not 

 found it necessary to alter his original dis- 

 cussion. 



lo is a small genus of large aquatic snails 

 confined to the Tennessee River system and 

 mainly to that part vfhich lies upstream from 

 Chattanooga. On looking over a representa- 

 tive collection of these shells one is immediately 

 impressed with the great variation in their 

 spinosity; some are absolutely smooth, and 

 there is every intergrade from these to shells 

 on which the spines are nearly or quite as 

 long as the radius of the whorl. In addition 

 there is a considerable range in the variates 

 which one is tempted to call " ordinary " — 

 general size of adults, globosity or shell index, 

 color, and so forth. Adams examined chiefly 

 the variation in shell diameter, globosity and 

 spinosity. This he did by making careful meas- 

 urements of large collections from stations 

 throughout the range of the genus. Data for 

 variation curves are given not only in abso- 

 lute frequencies, but in " frequencies reduced 

 to thousands " and in plotted curves. No data 

 are given for studying correlated variation, 

 nor has the variability of the characters been 

 analyzed by use of the statistical methods now 

 familiar, by reputation at least, to aU students 

 of evolution. There is probably ample justi- 

 fieatioh for this omission in the fact that it is 

 very diiEcult to be sure that a series is homo- 

 geneous with respect to age. At any rate, a 

 large amount of data is offered to any enter- 

 prising biometrician who may care to tackle it, 

 and the author seems to have gotten along 

 fairly well, as far as he has gone, without a 

 thorough mathematical analysis of the varia- 

 bility. 



An inspection of the curves shows that there 

 is a progressive change from the headwaters 

 of the various branches of the river system 

 downstream as follows : " From a greater diam- 

 eter of the shell to less ; from a high degree of 



1 Memoirs of the National Academy of Sciences, 

 Vol. XII., Part II., Second Memoir, 1915, pp. 

 1-184, 61 plates. 



globosity to one of a less degree ; from a spine- 

 less to relatively long spines; from a narrow 

 space between the spines to a wider space; 

 and from a relatively low spine index to one of 

 a high degree. The change from the smooth 

 to the spinose shell is relatively abrupt, as 

 shown by the modes, but there is a perfect series 

 of individual intergradations." The fact that 

 in the Holston River near Rogersville there are 

 smooth shells where one would expect from the 

 foregoing to find spiny ones will be referred to 

 later. 



The generally accepted belief among pale- 

 ontologists concerning the phylogenetic devel- 

 opment of spines is quoted from Beecher^ as 

 follows : 



The first species [of a group of animals] are 

 small and unornamented. They increase in size, 

 complexity and diversity, until the culmination, 

 when most of the spinose forms begin to appear. 

 During the decline extravagant types are apt to 

 develop, and if the end is not yet reached, the 

 group is continued in the small and unspeeialized 

 species which did not partake of the general tend- 

 ency to spinose growth. 



The author considers the possible eileets of 

 direct environmental action, hybridization 

 and other factors as explaining the distribu- 

 tion of the various shell types, but there are not 

 sufficient data at hand to reach a satisfactory 

 conclusion. Experimental work was started 

 but dropped because of lack of facilities. 

 However, the orthogenetic " law of ornamenta- 

 tion " just quoted, taken in connection with 

 stream history and the mixing of strains, 

 seems to explain many of the facts. 



What might be called phylogenetically young 

 streams are rapid. In such the phylogenetically 

 young lo developed, small and smooth. As 

 time went on the streams became older, less 

 rapid or with rapids further apart at the place 

 where lo started, but the streams continued 

 ever young at their heads where they worked 

 back into the land mass. lo became phylo- 

 genetically older at its place of origin and 

 progressed in its orthogenetic course toward 



- ' ' The Origin and Significance of Spines. A 

 Study in Evolution," Tale Bicentennial Publica- 

 tions, 1901, pp. 1-105. 



