1 42 THE POPULAR SCIENCE MONTHLY 



Italy and central Europe. Hyatt carefully traces out the complicated 

 genealogy of the related forms and describes their migrations, and his 

 monograph is beautifully illustrated by heliographic plates. In order 

 to carry out these studies, he visited the museums of Stuttgardt, Tu- 

 bingen, Wiirzburg, Munich, Zurich, Paris, Semur, the British and Geo- 

 logical Society Museums of London, and other places in Europe. 



In 1872-73, Hyatt lived in Wiirtemberg in order to study ammonites 

 and also variations and evolution of the fossil snail shells Planorbis 

 from the ancient Tertiary lake at Steinheim. As is well known, this 

 lake gradually rilled with gravel and limestone mud, and thus the later 

 shells lived at higher and higher levels until the lake became wholly 

 dry. Hyatt agrees with Hilgendorf that all of the species of Planorbis 

 found at Steinheim are descended from four varieties of Planorbis levis 

 which entered the lake in early times. At first hybrids were developed 

 between these four varieties, but as the original stocks diverged more 

 and more one from another, these hybrids died out. Hyatt finds that 

 in all four stocks there is at first a tendency to increase the spiral of 

 the shell due to a deepening of the lower at the expense of the upper 

 umbilicus, thus eventually producing more or less trochiform shells. 

 Hyatt states that the Steinheim shells develop similar species in many 

 separate and distinct genetic series, and these parallelisms he ascribes 

 to the fact that all lived in one and the same environment and were 

 subjected to similar external influences. His genealogical series differ 

 considerably from those of Hilgendorf, and the Steinheim shells must 

 be restudied by some unbiased investigator before we can be certain 

 of the facts in the controversy. Hyatt found that the young shells are 

 always smooth, but in one race transverse ridges appear on the outer 

 whorl and finally affect the inner whorls of their descendants. Un- 

 coiling also appears first in the outer whorl and finally the inner whorls 

 also uncoil, and in another stock a keel-like ridge forms first on the 

 outer and afterwards extends to the inner whorls. Thus these char- 

 acters are accelerated, i. e., appear earlier and earlier in the lives of the 

 descendants. Hyatt concludes that the modifications of the Steinheim 

 shells are due to the law of heredity with acceleration, and are not con- 

 trolled by natural selection, although natural selection may have caused 

 the dying out of the hybrids between the four original varieties. He 

 also believes that gravity produces modifications of structure,* and that 

 unfavorable conditions cause uncoiling, produce transverse ridges and 

 diminish the size of the shells. He states also that " the tendency to 

 earlier and earlier inheritance in successive generations is apparently 

 the result of disturbing and modifying agencies acting from without." 



For the last twenty years of his life Hyatt studied the mutations 

 and migrations of those most interesting of variable snails the Achati- 

 nelidse of the Hawaiian Islands, and, indeed, he was upon the point of 



