70 MEMOIRS NATIONAL ACADEMY OF SCIENCES. ivol.xii. 



begin to blend. * * * At present all mixed races are probably in condition of spurious 

 Mendelism or no Mendelism." 



This seems to be the most accurate description of what appears to be the condition in 

 lo, where the population is much mixed, assuming that inverse development of the degrees 

 of dominance may be taken as a rough estimate of the character of inheritance. Inversion 

 has its maximum development in the vicinity of Rogersville in the Holston, and a lesser degree 

 of development in the Upper Clinch and Powell. We might look upon these as different stages 

 or degrees in the process of fusion, a process which in all probability has been in progress for 

 thousands of years. 



Because the embryonic whorls of all the forms are smooth, carinate, or undulated, but 

 not spinose, we infer that the spinosity is a later degree of development and that the relatively 

 smooth shell represents approximately the ancestral form. Upon the bases of his very exten- 

 sive experimental studies upon moths, Standfuss ('96, pp. 110-117; 'OO-'Ol) claims that in 

 crosses the phylogenetically older "enforces its biological, morphological, and physiological 

 characters on its hybrid offspring to a greater degree than the younger." The supposed hybrid 

 lo with inverse development of spinosity may then be looked upon as the reassertion of domi- 

 nance of the phylogenetically older smooth shell upon the phylogenetically younger and spinose 

 kind. Thus inversion might be considered as a case of reversion to the ancestral form. 



Except in the case of specialized shells which are spinose throughout their postembryonic 

 development, spinosity develops at a variable age, from a very early age onward to very late 

 in life. In such shells as lyttenensis, paulensis, and loudonensis the shell is fairly large (class 

 two or three) before spines develop. Spinosity in such shells is clearly a later development 

 and would generally be considered of later phylogenetic development. Davenport ('08, pp. 

 60-61) claims that: 



A progressive variation, one which means a further stage in ontogeny will dominate over a condition due to an 

 abbreviation of the ontogenetic process, or a condition less highly developed than the first. The more developed 

 condition dominates over the less developed. 



From this standpoint the spinose shells would be expected to be dominant over the smooth, 

 but this is contrary to what we find in the case of the shells showing inverse development. 

 Thus inverse development agrees with Standfuss's law of the dominance of the phylogenetically 

 older over the younger. It may be possible that Standfuss's law applies to the earlier stages 

 of crossing and that later Davenport's law applies, because there are reasons (to be consid- 

 ered later) which suggest that the greater degree of inversion seen in the Holston shells may 

 be due to the relatively recent date at which the smooth and spinous shells were thrown together 

 by a change of drainage. As has been previously shown, in passing from the headwaters down- 

 stream there is a progressively earlier acquirement of spinosity; spinosity may be said to be 

 "crowded back" or acquired earlier and earlier until it occupies the entire postembryonic 

 development of the shell as in spinosa and its allies. We may even go further and note that 

 the old-fashioned, smooth shells have taken to the hills, to the headwaters, where isolated, they 

 have survived, whUe farther downstream on the lower lands the ascendant spinose forms 

 are abundant and have an extensive range; a story often paralleled in the history of the 

 human races. 



It has long been known to palaeontologists that the earUest known forms of animals are 

 often small, without spines, or other forms of ' 'ornamentation." This law has been formulated 

 by Beecher ('01, p. 99) as follows: 



The first species [of a group of animals] are small and unomamented. 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 unspeciaUzed species 

 which did not partake of the general tendency to spinose growth. 



In the family Pleuroceridse spinosity reaches its extreme development in lo, but none equal 

 or sm-pass that of the large individuals of lovdonensis and turrita. These shells, both in size 

 and sculpture, thus harmonize with Beecher's law. In this connection, however, mention 

 should be made of the fact that the size of the shell influences the size of the spines and that large 

 shells have large spines, and live in the large rivers. 



