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 Characteristic" 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 nautiloids 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 globiatum, 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 earlier than that in which it first developed. Indeed, it is one of the classic instances of an acquired character, and one of the best established cases of this sort in the whole field of zoology.
In order to establish these interesting facts Hyatt was obliged carefully 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 Arietidæ," 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 progressive forms migrated back again into the northeastern Alps and thence during Bucklandian and later times into parts of Germany,