I40 INTERESTING CASE OF INSECT MUTUALISM. 



I may add that a few years ago Mr. S. L. Hinde came 

 across some of these large frog-hoppers in British East Africa, 

 which not only produced the mimic showers, but also in a very 

 remarkable way mimicked flowers. With their wings closed, 

 these adult insects cluster round the narrow leaves of the trees, 

 the tips of their closed wings touching the leaf, and the head 

 slightly away from it. In this position the group presents much 

 the appearance of a branch of broom in flower. 



(Read, July 6. 1917.) 



Theories of Cosmogony.— Until the beginning of 



the present century the nebular hypothesis of Laplace held a 

 unique position as a tentative explanation of the origin of plane- 

 tary systems. The hypothesis had, it is true, been forced to 

 undergo many changes of detail, but its essential doctrine, that 

 increasing rotation was the primary cause of the birth of 

 satellites, remained almost undisputed. In recent years the 

 position of this hypothesis has been challenged by speculations 

 based ultimately upon the conception of tidal forces providing 

 the re(|uired tendency to separation, the most complete and 

 definite of these speculations being found in the planetesimal 

 hypothesis of Moulton and Chamberlain. Mr. J. H. Jeans, 

 M.A., F.R.S., recently read before the Royal Astronomical 

 Society a paper embodying the results of 'his mathematical inves- 

 tigation of the changes in a mass of matter as the tidal forces 

 acting on it continually increase. The paper is published 

 in the Society's Memoirs,'^ and in it the writer discusses 

 the tenability of the tidal theory of planetary evolution, arriving 

 at the following conclusions. The normal binary star formation 

 cannot be explained as the result of tidal action ; the genesis of 

 such systems must be ascribed to rotation. In regard to spiral 

 nebulae, 'however, there are certain features which can be ex- 

 plained in terms of tidal action, but on the whole it seems 

 impossible to reconcile this explanation with the known facts 

 of astronomy. The genesis of our own solar system may well 

 be attributed to tidal action, for the theory makes no impossible 

 or improbable demand here : at the same time, the origin which 

 seems most probable is not that of the planetesimal hypothesis. 



*71 [I], 1-48. 



