Definition of と言うと
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(conj) if one were to speak of ..., then certainly; if it were the case that ..., then certainly; if it were a ..., then certainly; phrase used to indicate the inevitability of what follows it (based on what precedes it)
ダーウィンというと「種の起源」を連想する。
We associate the name of Darwin with The Origin of Species. - so that means (when used in sentence-initial position)