8 
New York City. This species, though strikingly distinct from 
the Citrus species, readily crosses with them, indicating a much 
closer relationship, from a genetic standpoint, than species in 
genera often do, since in many cases the species of a genus will 
not cross. 
Striking examples of herbaceous perennial and woody species 
that occur in the tropics or sub-tropics (under hot-climate condi- 
tions) as well as in the cool-temperate are Tripsacum dactyloides, 
Tillandsia usneoides, Pteridium aquilinum, Osmunda cinnamomea, 
Cystopterts fragilis, Parthenocissus quinquefolia, Leitneria flori- 
dana, Daphniphyllum humile, Asimina triloba, Fontanesia philly- 
raeoides, Trichomanes radicans, Nymphaea odorata, Prunus 
serotina, and Polypodium poly podiotdes. 
In the light of present-day geological and genetic theories 
and investigations the most obvious interpretation of the facts 
say above is as follow 
. Certain groups a ei were already so genetically con- 
ae when the climatic changes toward cooler temperatures 
took place, that most of them could live under these changed 
conditions as well as under the previous state. Possibly the 
Salicaceae, Equisetaceae, and Ephedraceae belong to these groups. 
2. Other groups were completely destroyed in these regions 
and restricted to the tropics. To such a class, the Palmaceae 
and the Artocarpaceae possibly belong. 
3. Through mutations occurring in regions favorable both for 
increasing their own numbers and for aiding in their north and 
south distribution, the plants of the second group have been 
slowly emigrating into colder regions—in a sense coming back to 
the geographical regions in which their groups once thrived. 
But obviously comparatively few of the members of this group 
have or have had such favorable conditions for aiding in the 
establishment and distribution of their mutant types. Even 
with the utmost aid and ingenuity of man, the distribution of the 
most desirable and most improved varieties of cultivated plants 
takes many years, and often the places to which they go are un- 
favorable to their permanent establishment. Centuries elapsed 
before Zante “‘currants’’ moved far from Greece, and only 
recently Citrus decumana (C. grandis of Swingle) has been estab- 
lished away from the tropics. If a mutant able to survive 
