How to write in greek with xelatex polyglossia

Question

I'm using TeXshop on Mac. I'm writing an article in french using Xelatex and I need to type some sentences in ancient greek, so i tried :

\documentclass[12pt,a4paper]{article}

\usepackage{fontspec}

\usepackage{xunicode}

\usepackage{polyglossia}

\newfontfamily\greekfont[Script=Greek]{SPIonic}

\setmainlanguage{french}

\setotherlanguage[variant=ancient]{greek}

but it does not work. Here's what I get:

Package fontspec Warning: Font "SPIonic" does not contain requested Script
(fontspec)                "Greek".

(/usr/local/texlive/2018/texmf-dist/tex/latex/polyglossia/gloss-french.ldf)
(/usr/local/texlive/2018/texmf-dist/tex/latex/polyglossia/gloss-greek.ldf
(/usr/local/texlive/2018/texmf-dist/tex/latex/polyglossia/xgreek-fixes.def))
(./essai.aux) (/usr/local/texlive/2018/texmf-dist/tex/latex/tipa/t3cmr.fd)
Overfull \hbox (6.95477pt too wide) in paragraph at lines 20--21
\TU/lmr/m/n/12 différent, soit à un seul centre, soit à plusieurs (épicycle). [
]\TU/SPIonic(0)/m/n/12 metabasis
[1] [2] [3] (./essai.aux)
xdvipdfmx:fatal: Cannot proceed without the font: /Library/Fonts/SPIonic

Output file removed.
 )
(see the transcript file for additional information)
Error 256 (driver return code) generating output;
file essai.pdf may not be valid.
SyncTeX written on essai.synctex.gz.
Transcript written on essai.log.

I guess I have to chose another font but I don't know which font I can use to type in greek. I thought polyglossia and fontspec packages would do the job automatically.