108 
Here under the constant stress of changed conditions in the en- 
vironment, principally perhaps, by the chemical affinities contained in 
the food and atmospheric conditions, the progeny of the newly estab- 
lished colonies become modified wholly or in part as the adjustment 
of the organism to its environment demands and requires. 
This manner of distribution by the drainage system has been 
going on for ages, and is still scattering the slow-moving creatures 
over the face of the land, and it is not strange and no wonder that 
we meet with similar shells, high up on the mountains, low down in 
the valleys, far up in the cold north, away down in the hot south, back 
in the effete east, out in the wide west, as well as on the beautiful 
islands of the sea. 
