1885.] 
Correspondence. 
56r 
faTtW n £aCh ° f C ? Urt f y to remind Mr. Dawson of the open 
tact that parwm is also the writer on the Movements and Habits 
of Climbing Hants, of a monograph on the Cirripedia, on the 
a ;r iSatl0 i n ° f 9 rchlds ’ on the Expression of Emotions in Man 
and Animals, and on the Variation of Animals and Plants under 
omestication,— all biological works. Mr. Dawson will also 
, n . c ’. 1 " e . re ^ ds treatise on the Formation of Mould, that 
‘S is distinctly a biological work, since that formation is traced 
to the action of an animal. 
F. E. S. 
THE PROPORTIONS OF THE SEXES. 
A correspondent in your August issue (p. 497) suggests some 
very interesting experiments in incubation. In insects the sex 
°t an individual seems not to be determined until towards the 
end of the larval stage. At least entomologists who rear moths 
and butteiflies contend that when the food-supply is scanty the 
majority of perfect insects produced are males, whilst if food is 
superabundant females preponderate. In the human species the 
iespective proportions of male and female children are neither 
alike in all countries. Unless my memory deceives me the pro- 
portion of male births in England is increasing, and the total 
bnth-rate during the last few years decreasing, — both matters for 
congratulation. 
Zero. 
THE BALANCE OF NATURE. 
That “ numbers invite enemies,” as your correspondent H. 
Browne says, is undeniable. The pests which ravage the vege- 
table world seem to have only assumed formidable proportions 
since fields, plantations, and orchards afforded them scope. 
There can be no doubt that Aphides are far more plentiful in 
England now than they were half a century ago. The rose- 
growers whom I knew in my young days had little trouble with 
“ green-fly,” but their successors have to play away with soap- 
suds and water, as if upon a conflagration, to keep their rose 
trees clean. This year a black Aphis is in fashion. 
Richmond. 
