§ 80. The Latin of the fourth period and the Greek agree in retaining, in many cases, original inflexions rather than adopting the English ones; in other words, they agree in being but _imperfectly incorporated_. The phænomenon of imperfect incorporation is reducible to the following rules:-- 1. That it has a direct ratio to the date of the introduction, i.e., the more recent the word the more likely it is to retain its original inflexion. 2. That it has a relation to the number of meanings belonging to the words: thus, when a single word has two meanings, the original inflexion expresses one, the English inflexion another--_genius_, _genii_, often (_spirits_), _geniuses_ (_men of genius_). 3. That it occurs with substantives only, and that only in the expression of number. Thus, although the plural of substantives like _axis_ and _genius_ are Latin, the possessive cases are English. So also are the degrees of comparison for adjectives, like _circular_, and the tenses, &c. for verbs, like _perambulate_. § 81. The following is a list of the chief Latin substantives introduced during the latter part of the fourth period; and preserving the _Latin_ plural forms-- FIRST CLASS. _Words wherein the Latin plural is the same as the Latin singular._ (a) _Sing._ _Plur._ | (b) _Sing._ _Plur._ Apparatus apparat-us | Caries cari-es Hiatus hiat-us | Congeries congeri-es Impetus impet-us | Series seri-es | Species speci-es | Superficies superfici-es. SECOND CLASS. _Words wherein the Latin plural is formed from the Latin singular by changing the last syllable._ (a).--_Where the singular termination -a is changed in the plural into -æ_:-- _Sing._ _Plur._ | _Sing._ _Plur._ Formul-a formul-æ | Nebul-a nebul-æ Lamin-a lamin-æ | Scori-a scori-æ. Larv-a larv-æ | (b).--_Where the singular termination -us is changed in the plural into -i_:-- _Sing._ _Plur._ | _Sing._ _Plur._ Calcul-us calcul-i | Polyp-us polyp-i Coloss-us coloss-i | Radi-us radi-i Convolvul-us convolvul-i | Ranuncul-us ranuncul-i Foc-us foc-i | Sarcophag-us sarcophag-i Geni-us geni-i | Schirr-us schirrh-i Mag-us mag-i | Stimul-us stimul-i Nautil-us nautil-i | Tumul-us tumul-i. Œsophag-us œsophag-i | (c).--_Where the singular termination -um is changed in the plural into -a_:--