\newcommand defined for declarative formatting works for \it but not \textit or \emph

Question

I defined this \newcommand for binomial genus/species names, which should be in italics:

\newcommand{\taxon}[1]{{\it #1}}

For whatever reason, it only renders the relevant text in italics if I use \it; \emph or \textit do not work (the text is unchanged).

Is there a reason for this strange behaviour? \textit and \emph work perfectly well if I apply them directly.

Note that \textit takes an argument. Have you written \textit{#1}?
I'm curious: \it is deprecated since 1993 (release of LaTeX2e). Which documentation told you to use it?

Heiko Oberdiek · Answer 1 · 2012-08-27 06:41:44Z

Update: \it is deprecated in LaTeX2e, therefore using \itshape instead.

\itshape is the declarative form. At this time the text is in italics until the current group ends. \textit sets the argument in italics:

{\itshape ...}
\textit{...}

\emph emphasizes the argument, whereas \em is the declarative form.

The macro definitions for these variants:

\newcommand*{\taxon}[1]{{\itshape #1}}
\newcommand*{\taxon}[1]{{\em #1}}
\newcommand*{\taxon}[1]{\textit{#1}}
\newcommand*{\taxon}[1]{\emph{#1}}

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