
OUR BOOK SHELF, 65 

as propellers: the same causes which determined the position of 
the legs would also affect the wings. Thus a division of labour 
would be effected ; the branchize on the thorax would be devoted 
to locomotion, those on the abdomen to respiration. This would 
tend to increase the development of the thoracic segments, already 
somewhat enlarged, in order to receive the muscles of the legs.” 
The necessity of wings would occur only when fresh breeding 
grounds were required, and their development would, therefore, 
be “relegated to a late period of life; and by the tendency to 
the inheritance of characters at corresponding ages, which Mr. 
Darwin has pointed out, the development of wings would thus 
become associated with the maturity of the insect.” As the 
characters of the species became more and more differentiated 
from the original form, in order to suit various conditions of life, 
genera and orders would result. Sir John Lubbock at the same 
time admits that the earliest fossil remains, “though in some re- 
spects less specialized than existing forms,” represent “as well 
characterized insects as any now existing, and we have no reason 
for supposing that the other orders of insects were derived from 
either the Neuroptera or Orthoptera.” It appears to me that an 
objection to this theory is found in the fact that locomotion is now 
universally effected by the abdominal muscles of these aquatic 
larvee, assisted only ‘by feeble strokes of the legs ; the thoracic and 
abdominal plates are not used for this purpose, even when the 
latter do not vibrate during active respiration, and might, there- 
fore, be supposed to subserve a double function, whereas the 
plates on the thorax are to some extent contributory to the respira- 
tory function. It is not, however, desirable to criticise too severely 
a theory which has at least the merit of ingenuity, and which ema- 
nates from so high an authority, until more is known concerning 
the early forms of insects, especially in their aquatic condition. 
Much more might be written concerning the subject of this 
paper. Its intention, however, is merely to record a few discon- 
nected thoughts which have arisen during the study of this interest- 
ing family of insects. 

OUR BOOK SHELF. 
BritisH FREesH-Water Atca. By M.C. Cooke, M.A., LL.D., 
A.L.S. 1882. London: Williams and N Ofgate: OVO, ANG. I., 
pp. 28, with rr coloured plates. 
The first number of this excellent work by Dr. Cooke has just 


