The first point of similarity between the two species, Saporta argued, was dentition; the arrangement of teeth in the mouth of humans and simians , he said, “seems to denote an exclusive link with the Monkeys of the old continent“. The second similarity was decidedly more risqué, namely “female menstruation and, as a corollary, the odour which makes women attractive to many monkeys“.
Alongside other similarities – including bald foreheads and the direction of hairs on the forearm - analogous dentition in apes and humans was something which Darwin had already discussed at length in Descent of Man, published the previous year. Nowhere in the body of Descent, however, did Darwin mention the menstrual cycle or, more specifically, scent-based sexual attraction between monkeys and humans.
That is not to say, however, that Darwin was ignorant of the issue. Embedded in a footnote and suitably “veiled in Latin”(just as his cautious editor, John Murray, had recommended) Darwin refers to the role of smell – “odoratu” – in the courtship processes of humans and apes alike. By presenting this sort of content in Latin, Darwin was able to protect the sensibilities of his general readership without depriving his learned audience of a piece of evidence which corroborated his theory of the shared heredity of man and ape. 
Interestingly, even once shrouded in a decorous cloak of Latin, Darwin remained sketchy on detail: “Males from various species of mammals”, he said, “clearly distinguish anthropomorphous females from male. First, I believe, by smell, then by appearance.” In conceding that the topic in question was “turpius” (i.e. unseemly) in character, one can justifiably conclude that the odaratu to which Darwin referred was similar in kind to that observed and communicated by Saporta.
What was it about this topic which rendered it taboo? Evidence from elsewhere in the correspondence suggests that Darwin felt uncomfortable discussing the topic of menstruation, at least in public. When the medical director of Wakefield Asylum wrote to him to report the case of a woman inmate who believed that she was pregnant until “a distinct return of menstruation”, for example, he replied that while “truly wonderful” the topic could not feature “in any work not strictly medical”. “Perhaps I may manage to give it wrapped up,” he said, “or anyhow allude to it”.
Darwin might also have felt uncomfortable about the potential impropriety of drawing parallels between the courtship processes of man and ape. As Gillian Beer has noted, in the wake of the publication of Descent there existed a certain “sexual distate…for many Victorians in the idea of kinship with other animal species”. 
Distaste at the sexual implications of Darwin’s theory of shared heredity was played out at length in the popular press where images such as The Descent of Man and That Troubles Over Our Monkey Again encapsulated Victorian society’s sense of unease with the notion that humans had, at some stage, shared their beds with apes. 
Gaston de Saporta’s correspondence helps bring to light the strategies which Darwin could (and indeed did) employ in order to pitch his work both as a palatable read for his respectable popular audience and – at the same time – a robust and convincing work of science. It also helps us to better understand hierarchies of impropriety in Victorian Britain. Thus, while Darwin deemed certain risqué topics to be suitable for a learned (if not popular) readership, other subjects – including menstruation – remained taboo in all but the most private of contexts.
 For more on this subject see Gowan Dawson, Darwin, Literature and Victorian Respectability, (Cambridge, 2007), p. 37.
 Gillian Beer, ‘Forging the Missing Link’ in Open Fields: Science in Cultural Encounter, (Oxford, 1999), pp. 131 – 132.
 Gordon Thomson, That Troubles Over Our Monkey Again (Fun 16, 1872) featured Darwin as half-man half-monkey, complete with conspicuously erect tail. George du Maurier, Descent of Man (Punch 64, 1873) complete with lewd reference to a sexual encounter between an ape and the featured character’s ancestor.