  We investigate how the qualitative structure of Doppler peaks in the angular
power spectrum of the cosmic microwave anisotropy is affected by basic
assumptions going into theories of structure formation. We define the concepts
of ``coherent'' and ``incoherent'' fluctuations, and also of ``active'' and
``passive'' fluctuations. In these terms inflationary fluctuations are passive
and coherent while topological defects are active incoherent fluctuations.
Causality and scale invariance are shown to have different implementations in
theories differing in the above senses. We then extend the formalism of Hu and
Sugiyama to treat models with cosmic defects. Using this formalism we show that
the existence or absence of secondary Doppler peaks and the rough placing of
the primary peak are very sensitive to the fundamental properties defined. We
claim therefore that even a rough measurement of the angular power spectrum
$C_l$ shape at $100<l<1500$ ought to tell us which are the basic ingredients to
be used in the right structure formation theory. We also apply our formalism to
cosmic string theories. These are shown to fall into the class of active
incoherent theories for which one can robustly predict the absence of secondary
Doppler peaks. The placing of the cosmic strings' primary peak is more
uncertain, but should fall in $l\approx400-600$.
