400 
POPULAE SCIENCE EEYIEW. 
fessor Owen concludes tliat the Echidna may properly be grouped along 
with the Other monotremes in the division Aplacentalia. 
How Birds Fly. — Birds, like insects, can progress by flight iu three 
distiuct ways : (1) Flight without locomotion ; (2) with locomotion 
and beating of the wings : and (3) without beating of the wings. The 
second is the ‘ method most generally adopted, and appears to require 
less labour, the rapidity of the wing movements being less than in 
the other modes. The cause of this is, that the wings experience no 
resistance in ascending, TVTien a bird is about to depress its wing, this is a 
little inclined from before backwards ; but the movement is accompanied by 
a rotation of several degrees round the anterior edge, so that the wing 
descends more in front than behind, and the descending movement is trans- 
feiTed more and more backwards, at the same time that the wing becomes 
more and more inclined, so as to give simultaneously an ascending and 
accelerating horizontal advance. Towards the close of the movement, a fresh 
rotation takes place round the anterior margin of the wing, but in the oppo- 
site direction, so as to bring the posterior part on a level with the anterior, or 
even a little below it. This also produces an ascending movement. When the 
wing has completely descended, it is both further back and lower than at the 
commencement of the movement ; but, as at this commencement, its jDosterior 
part is a little lower than its anterior. It is then raised in this position.— 
Vide a memoir by M. E. Liais, in Conrptes JRendus, November. 
Tempjerature required for Incubation of Hen Eggs. — M. Dareste narrates 
the results of some curious experiments which he made to determine the 
lowest temperature at which hen eggs could be hatched, or rather at which 
embryos could be developed. He has found that incubation may go on at as 
low a temperature as 30° of the centigrade thermometer. The embryos, however, 
which are produced at a temperature of from 30° to 34° are very slowly deve- 
loped. None of them ever emerge from the shell, as all perish before arriTmg 
at maturity. Some of them die earlier than others ; but none of them reach 
that period when the embryo surrounds the vitellus, !Most of the embryos 
which underwent incubation in this manner were monsters. Some presented 
anomalies of the head, of which the Cyclops variety was most frequent; 
others presented a double heart ; and iu some cases both these abnormal 
conditions were present together.— Bendus, January 9tL 
T?ie Metarnorphoses of Fishes. — The subject of the metamorphoses of 
animals is one of great interest to the zoologist, and therefore the newly- 
announced facts recorded in the memoir presented by M. Agassiz to the 
French Academy will produce considerable excitement in the Natural History 
world. Although M. de Quatrefages in his recent essay admitted that certain 
fish imdergo slight alterations in point of structure, subsequent to the ab- 
sorption of the vitelline sac, yet he had (at the time of writiug his work) no 
idea that such extreme alterations occur, and are of such general occurrence, 
as are now asserted to take place, Agassiz, who may fairly be regarded as 
the most distiuguished living ichthvologist, states that he has observed as 
important metamorphoses among fish as those which occur in the case of 
reptiles. He has seen certain young fish, which apparently belonged to the 
families Gadidce and Blennidce, gradually assume the forms characteristic of 
the groups Labridcc and Lophidee. He thinks, too, that he can demonstrate 
