ALPHEUS HYATT, 18S8-1902 141 



Hyatt believed in the inheritance of acquired characters. By such 

 he understood modifications which appear in adult or late stages of 

 growth, and are due to the influence of external conditions and not 

 caused by heredity. Probably the most interesting and important 

 paper which Hyatt wrote is his " Phylogeny of an Acquired Charac- 

 teristic " published in the Proceedings of the American Philosophical 

 Society in 1893. In this he shows that at first the young shell in 

 the nautiloids is nearly straight, but soon the shell bends around 

 and grows over the outer side of its older part. The cross section of 

 the young shell before it overgrows itself is round, but when it presses 

 against its first whorl it is squeezed inward on one side, or impressed. 

 This impressed region is due entirely to the pressure of the shell in 

 overgrowing its older whorl, for in the Silurian and Devonian nauti- 

 loids the shell does not become impressed until it actually comes in 

 contact with the older whorl, and is thus squeezed inward on its inner 

 side as it passes around the outer side of its older part. That this is 

 due solely to pressure is shown by the fact that when these Silurian 

 and Devonian forms uncoil the impressed zone disappears at once in 

 the uncoiled part, the cross-section of which is round as in the young 

 shell before it grew over its first-formed whorl. 



In the Carboniferous species Coloceras glohiatum, however, the 

 impressed zone appears in the young whorl long before it has touched 

 and grown over its first whorl, and in the Jura most of the nautiloids 

 develop an impressed zone before the shell touches its first whorl. As 

 Hyatt states, the character has become hereditary and appears at an 

 earlier stage than in the Devonian ancestors. There is also a quicker 

 development of the coiling tendency in Jurassic shells and still more 

 so in the Cretaceous. 



It is hard to escape the conclusion that this is actually an acquired 

 character which becomes hereditary, and finally appears at a stage ear- 

 lier than that in which it first developed. Indeed, it is one of the 

 classic instances of an acquired character, and one of the best estab- 

 lished cases of this sort in the whole field of zoology. 



In order to establish these interesting facts Hyatt was obliged care- 

 fully to crack apart a large collection of nautiloid shells to make a 

 microscopic study of their earliest whorls. 



In 1889, Hyatt published his final paper upon the " Genesis of the 

 Arietidae," a large family of the ammonoids. He agrees with Neumayr 

 that three of the four great branches of this family are descended from 

 a single species, Psiloceras planorbe, which was itself derived from P. 

 caliphyllum of the northeastern Alps. The race then migrated into 

 Italy, south Germany, and the Cote-d'Or in which last place new pro- 

 gressive forms migrated back again into the northeastern Alps and 

 thence during Bucklandian and later times into parts of Germany, 



