From the 6th of May, when she started operations, to the 25th of May, the date at which her laying ceased, the Osmia occupied seven Snail-shells in succession. Her family consists of fourteen cocoons, a number very near the average; and, of these fourteen cocoons, twelve belong to males and only two to females. These occupy the seventh and thirteenth places in chronological order. Another, between the 9th and 27th of May, stocked six Snail-shells with a family of thirteen, including ten males and three females. The places occupied by the latter in the series were numbers 3, 4 and 5. A third, between the 2nd and 29th of May, colonized eleven Snail-shells, a prodigious task. This industrious one was also exceedingly prolific. She supplied me with a family of twenty-six, the largest which I have ever obtained from one Osmia. Well, this abnormal progeny consisted of twenty-five males and one female, one alone, occupying place 17. There is no need to go on, after this magnificent example, especially as the other series would all, without exception, give us the same result. Two facts are immediately obvious. The Osmia is able to reverse the order of her laying and to start with a more or less long series of males before producing any females. In the first case, the first female appears as number 7; in the third, as number 17. There is something better still; and this is the proposition which I was particularly anxious to prove: the female sex can be permuted with the male sex and can be permuted to the point of disappearing altogether. We see this especially in the third case, where the presence of a solitary female in a family of twenty-six is due to the somewhat larger diameter of the corresponding Snail-shell and also, no doubt, to some mistake on the mother's part, for the female cocoon, in a series of two, occupies the upper storey, the one next to the orifice, an arrangement which the Osmia appears to me to dislike. This result throws so much light on one of the darkest corners of biology that I must attempt to corroborate it by means of even more conclusive experiments. I propose next year to give the Osmiae nothing but Snail-shells for a lodging, picked out one by one, and rigorously to deprive the swarm of any other retreat in which the laying could be effected. Under these conditions, I ought to obtain nothing but males, or nearly, for the whole swarm.