Prof. J. Wood-Mason on a Viviparous Caddis-fly . 139 
assumes the fundamental fact on which the theory of segre- 
gation rests. All that is wanting is its recognition as a 
universal principle on which all permanent divergences, 
whether varietal or specific, necessarily depend. In the 
formation of domestic variations it is fully recognized ; for he 
says, u It is only by isolation and pure breeding that any 
specially desired qualities can be increased by selection ” 
(p. 99). If experimental biology shows this to be a constant 
law, is there any good reason for not applying it in the general 
theory of organic evolution? Seeing it is admitted that arti- 
ficial selection, unaided by isolation, is of no avail in pro- 
ducing divergent races, how can it be claimed that natural 
selection, unaided by isolation, is of any avail in producing 
varieties and species ? Again, as in domestication the segre- 
gate breeding of other than average forms always produces 
divergence, have we any reason to doubt that, when the same 
process takes place in the grouping of organisms in a natural 
state, the result will also be divergence ? 
The discrepancies to which I have referred are, it seems to 
me, due to deficiencies in the theory which Mr. Wallace 
maintains in common with many others. These problems 
that drive the exclusive utilitarian into various inconsisten- 
cies, can, I am convinced, be consistently explained by the 
theory of Divergence through Segregation. 
26 Concession, Osaka, Japan. 
XV. — On a Viviparous Caddis-fly . By J. WOOD-MASON, 
Superintendent of the Indian Museum, and Professor of 
Comparative Anatomy in the Medical College of Bengal, 
Calcutta. 
Some years ago, while studying a series of transverse sections 
through the body of a Trichopterous insect I had captured at 
the dinner- table lights, I noticed that the abdomen was 
crammed from end to end with partially developed ova. On 
the 25th October last I caught a second specimen of the same 
species, which also proved to be a gravid female. .Remem- 
bering my former observation, and having often observed 
that gravid females of the viviparous forms of Muscidse bring 
forth their young on falling accidentally into the spirit of the 
dissecting-dish, I threw the insect alive into a liqueur-glass of 
whiskey that happened to be ready at hand. The moment that 
