 Aristotle believed that the fundamental difference between male and female animals was a different capacity to concoct excess blood in the body into semen through a process that involved the transmission of heat to the blood; ultimately this difference was caused by a difference in heat, or the capacity to transmit heat, in the hearts of males and females (Marinella cites History of Animals IX, but there is considerable evidence for this view in the Generation of Animals as well; see, for example GA IV. 1 766a31–6). That is, the distinction between male and female animals resides in their heart, which is the source of natural heat, such that women are less capable of producing vital heat.