572 
Sinnott and Bailey. — The Origin and 
Evidence from fossil plants seems to indicate clearly that the climate of 
Europe, North America, and the Arctic regions, very uniform and equable 
at the beginning of the Tertiary, grew distinctly colder during the course of 
that epoch, and also that it became clearly differentiated into zones with 
decreasing temperature from south to north. This is shown by the occur- 
rence within the Arctic Circle in the later Cretaceous of tropical types, 
which were succeeded in the middle Tertiary by a distinctly temperate 
vegetation, this in turn giving way to the impoverished Arctic flora of 
to-day. Conditions changed less rapidly in more southern latitudes, and 
the ancient flora became slowly modified into its present temperate form. 
Throughout the Tertiary the climate of the great northern land-mass seems 
to have suffered a progressive refrigeration. We have already called 
attention to the fact that herbs are much better able than woody plants to 
withstand a climate with extremes of cold, and this suggests the hypothesis 
that the herbaceous type of vegetation has been developed in the north 
temperate zone mainly as an adaptation to this progressive refrigeration of 
the climate. A more detailed discussion of the various factors which have 
led to the evolution of herbs will be found in the latter part of this paper, 
where evidence will also be brought forward that the majority of herbaceous 
genera have arisen in the northern hemisphere and have spread thence over 
the rest of the world. 
3. The Herbaceous Element in various Insular Floras. 
A. Endemism as a Criterion of Antiquity . 
Evidence of much value in helping to solve the problem which 
confronts us may be drawn from an analysis of the composition of any 
flora known to be very ancient. In the study of all floras one of the most 
certain criteria of antiquity to which we may appeal is the extent of 
endemism displayed. It is generally admitted that the endemic species in 
a flora, those which are peculiar to it and are found in no other region, are 
for the most part more ancient than the non- endemic element, for they must 
either have had their origin in the region — a process usually requiring a long 
period of time — or else they must be remnants of an older vegetation which 
has elsewhere become extinct. Endemic genera and finally endemic families 
are in this way regarded as progressively more ancient portions of the flora. 
Although this rule is in general a pretty safe one to follow, it cannot be 
applied too rigidly in an attempt to determine the comparative age of herbs 
and woody plants, for herbs seem to change much more rapidly, and hence 
become sooner endemic. We have seen, for example, that the indigenous 
dicotyledonous flora of the eastern United States is composed of 77 per cent, 
herbaceous forms and 33 per cent, woody ones. These figures are probably 
about the average for the combined flora of southern Canada, the United 
