  We examine the possibility of evolution with redshift in the mean rest-frame
ultraviolet (UV; <4500A) spectrum of Type Ia Supernovae (SNe Ia) sampling the
redshift range 0<z<1.3. We find new evidence for a decrease with redshift in
the strength of intermediate-mass element (IME) features, particularly Si II
and to a lesser extent Ca II "H&K" and Mg II blends, indicating lower IME
abundances in the higher redshift SNe. A larger fraction of luminous, wider
light-curve width (higher "stretch") SNe Ia are expected at higher redshift
than locally, so we compare our observed spectral evolution with that predicted
by a redshift-evolving stretch distribution (Howell et al. 2007) coupled with a
stretch-dependent SN Ia spectrum. We show that the sense of the spectral
evolution can be reproduced by this simple model, though the highest redshift
events seem additionally deficient in Si and Ca. We also examine the mean SN Ia
UV-optical colors as a function of redshift, thought to be sensitive to
variations in progenitor composition. We find that the expected stretch
variations are sufficient to explain the differences, although improved data at
z~0 will enable more precise tests. Thus, to the extent possible with the
available datasets, our results support the continued use of SNe Ia as
standardized candles.
