This article by Ariew is proving pretty interesting. Here's a quote explaining proximate causes.
"For example, to study how warblers migrate, one cites either the operation of the warbler’s physiology or the external conditions that trigger it. Mayr calls the former cause of warbler migration the ‘‘intrinsic physiological cause’’: ‘‘The warbler flew south because its migration is tied in with photoperiodicity. It responds to the decrease in day length and is ready to migrate as soon as the number of hours of daylight have dropped below a certain level’’ (p. 1503). Mayr calls the ‘‘triggering’’ cause of warbler migration the ‘‘extrinsic physiological cause’’: ‘‘the warbler migrated on the 25th of August because a cold air mass, with northerly winds, passed over our area on that day. The sudden drop in temperature and the associated weather conditions affected the bird, already in a general physiological readiness for migration, so that it actually took off on that particular day’’ (p. 1503) "Ariew goes onto illustrate two different types of questions that could be answered while remaining within Mayr's proximate cause zone.
There is a difference between the study of functional morphology and the study of development. Functional morphologists answer questions like ‘‘how does something operate?’’, e.g. ‘‘how do hearts contribute to circulation?’’, while developmental biologists ask questions like ‘‘how does something come to be?’’, e.g. ‘‘how do hearts come to develop out of embryonic cells?’’. These questions get distinct answers because the causal processes to which they refer are distinct; physiological causation is distinct from developmental causation.
Proximate explanations answer causal questions of individuals and the ultimate explanations answer questions about the prevalence and maintenance of traits in a population.