192 LINNEAN SOCIETY OF LONDON. 
primary specific distribution ; (2) it not ex y 
the secondary specific distinctions, or those trivial details of struc 
ich, while serving to distinguish one species nother, 
n 
re than a very small percentage of cases, although, when inter- 
crossing is prevented by the bar of sterility between species, 
that the theory of natural selection has been misnamed a theory of 
origin of species. It is, in truth, a theory of the origin of 
calls physiological selection, or in virtue of a variation taking place in 
the reproductive system in the direction of sterility (whether absolute 
0} 
species when all the individuals in that portion (or new variety) — 
would be absolutely sterile towards the rest of the species, while 
other specific characteristics, and entirely escapes the difficulty from 
intercrossing. It therefore relieves the theory of natural selection 
from all which it lies, in consequence of 
“ f 
_ Species. — The paper, “ On new species of the genus Metzgeria,” by 
William Mitten, A.L.S., was afterwards read in abstract. 
