594 
Sinnott and Bailey.— The Origin and 
It is probable that the earliest herbs were developed in mountainous 
regions, as it would be here that extremes of climate would first be felt. 
That decrease in temperature has indeed been the chief factor in the evolu- 
tion of herbs on the north temperate zone is indicated by the fact that 
families possessing woody forms which are able to withstand cold almost 
never include herbs. Thus the Salicaceae, Betulaceae, Fagaceae, Myrica- 
ceae, Empetraceae, Aquifoliaceae, Aceraceae, Ericaceae, Rhamnaceae, 
Tiliaceae, and Thymeleaceae, which comprise almost all the woody plants 
of the north temperate zone, have practically no herbaceous members. 
Being naturally hardy to cold, they have not been forced to adopt the 
herbaceous habit. The more susceptible families, on the other hand, which 
once flourished in northern regions, have either developed herbaceous 
representatives or been exterminated. 
Refrigeration of climate has doubtless produced herbaceous plants 
in mountainous regions throughout the globe as well as in the Polar land 
areas. The Andes, the highlands of Africa, and the Southern Alps of New 
Zealand have certainly been the centres of origin of many herbs. In the 
last-named region, the reduction of the genus Veronica, for example, from 
trees down through all degrees of dwarfing to perennial herbs may be very 
clearly traced. Many herbs of the tropical lowlands have apparently been 
produced in mountainous regions and then migrated downward. 
A very large number of herbs have doubtless arisen in arid regions 
also, springing up rapidly and producing seed during a rainy season and 
thus bearing the same relation to extremes of moisture that northern herbs 
bear to extremes of temperature. Many of the endemic herbs of South 
Africa, West Australia, and other regions which are subject to dry seasons 
of considerable length have in all probability been developed in this way. 
The stunting effect of desert conditions has resulted in an extreme paucity 
of trees in arid regions and in the reduction of most of the woody species 
to stunted undershrubs or suffruticose plants, some of which are practically 
perennial herbs. 
There is also a considerable element of characteristically tropical herbs, 
which seem to have been evolved in very warm regions and not to have 
come from the mountains. They were probably developed in response to 
variations in moisture consequent on the alternating wet and dry seasons, 
or for various other reasons. The genus Peperomia is a good example 
of such tropical herbs. 
A great body of herbaceous plants, originating for the most part in the 
tropics, have apparently not arisen because of extremes of either heat 
or moisture but from the acquisition of a rapidly climbing habit. The 
Cucurbitaceae, Convolvulaceae, Asclepiadaceae, and other families and 
genera are composed mainly of climbing plants, and in them there has 
been a progressive reduction in the amount of xylem and increase in bulk 
