Central Part of British North America. 223 
there are great expanses of desert plain, owing to the influence 
of the Cascade Range of mountains, which forms almost an un- 
broken wall 4000 feet in height, running parallel to the coast, 
and cutting off the moisture from the interior. In descending 
to the westward, therefore, from the Rocky Mountains, into 
the depression that intervenes between them and the Cascade 
Range, a belt of moist climate is met with where the winds that 
have passed over the coast-range first strike on the higher and 
interior range. Then follows a belt of dry climate, increasing 
in aridity as we proceed westward, and get more completely 
sheltered by the coast range, but on crossing which we at 
once get into the humid climate of the Pacific coast, with its 
wonderful development of forest growth. 
Having thus briefly sketched the leading physical pecu- 
liarities which influence the character of the vegetation in 
the region treated of, I shall next give a short outline of the 
manner in which authors have divided British North America 
into botanical areas, and endeavour to show the position which 
the country explored by the Expedition occupies among them, 
as indicated by the collections of M. Bourgeau and by the 
physiognomy of its vegetation. 
The collections made by M. Bourgeau were forwarded to 
England from time to time, and were duly received by Sir 
William Hooker at the Royal Botanic Garden at Kew. They 
consisted—Ilst, Of plant specimens prepared for preservation 
in the herbarium ; 2d, Seeds and roots of plants for culture, 
many of which have been successfully raised at Kew; 34d, 
Specimens of the vegetable products used in the country by 
the Indians, and which are preserved in the Kew Museum of 
Economic Botany. M. Bourgeau also made collections of 
insects and shelled mollusca, all of which were forwarded to 
the British Museum. 
Of the dried plants, there were in general twelve specimens 
of each species sent home, and the duplicate sets have been 
distributed to the yarious public herbaria in Europe and 
America, including that of our own university, each specimen 
having been named before its issue from Kew by Mr Black, 
curator of the Herbarium there, who prepared the list under 
the superintendence of Dr Hooker. 
