servations, therefore, apart from the more obvious object which I had in view, tends to confirm the opinion, that all the young of the same brood are of one sex; and this, again, if established, increases the evidence, that, in the above experiment, no single male may, by chance, have been present, and have escaped observation. This is, indeed, however, scarcely possible, because I examined each brood when they were at most a day or two old, and then isolated two or three specimens, all of which were indubitably females; and, indeed, except the last pair, they all proved fertile.
Considering, also, how much rarer the males are than the females, it is a strong fact in the same direction, that the only time I satisfactorily proved the formation of a male from an agamic egg, the whole brood, eighteen in number, were of that sex.
The fact is, therefore, to my mind, certainly established, that, at least as far as nine generations, the females of Daphnia can continue the species without the intervention of the male; but I by no means believe that the power is then exhausted. It must, indeed, be confessed that the last two generations of my Daphniæ were smaller and weaker than those that had gone before; but I am inclined to attribute this partly to the season (December), but principally to the fact, that certain small algæ, which had flourished all the summer in my tumblers, and had furnished an abundant and suitable supply of food to the Daphni, had died out, and thus left them without sufficient food.
As a general rule, I found that in the heat of summer young Daphniæ laid eggs for the first time (though, under the circumstances, "laying" their eggs is scarcely a suitable expression), when they were about three weeks old, and before they are full grown, or even sometimes a few days earlier; and after this they produced a fresh brood about every seven or eight days.
Although, however, they breed with so much rapidity, they live in confinement a considerable time; I even kept one large specimen by itself in a tumbler from the 21st of January to the 20th of May, when it died; and as it was full-grown at the earlier date, it must have lived for at least five months, and even then it may have died from disease, rather than from old age.
With reference to the homologies of the inner ephippial case, Prof. Leydig expresses no opinion. The explanation of it given by me has, however, lately been called in question by M. F. A. Smitt;[1] he confirms, indeed, in all respects, the accuracy of my descriptions; and, as also does Prof. Leydig, agrees with me in admitting the correctness of Straus' views as to the nature of the outer valve. I am happy to think also that the apparent difference between us has arisen entirely from his misunderstanding the perhaps too brief description of the structure given by me.
The valve of a Daphnia, being a projection of the skin, consists, of
- ↑ Nova Acta Reg. Societ. Scient. Upsal., Ser. 3æ., vol. iii.
