298 HOW CROPS GROW. 
degree departed from the original specific type. Thus 
nature puts the same limit to variation within a species 
that she has established against the mixing of species. 
Darwin’s Hypothesis, which is now accepted by many 
naturalists, is to the effect that species, as above defined, 
do not exist, but that new kinds (so-called species) of ani- 
mals and plants may arise by variation, and that all exist- 
ing animals and plants may have developed by a process 
of “natural selection” from one original type. Our ob- 
ject here is not to discuss this intricate question, but sim- 
ply to put the reader in possession of the meaning attach- 
ed to the terms currently employed in science—terms 
which must long continue in use and which are necessarily 
found in these pages.* , 
Genus, (plural Genera.)—In the language of anti-Dar- 
winianism, any set of oaks that are capable of reproducing 
their kind by seed, but cannot mix their seed with other 
oaks, constitute a species. Thus, the white oak is one 
species, the red oak is another, the water oak is a third, 
the live oak a fourth, and so on. All the oaks, white, red, 
ete, taken together, form a group which has a series of 
characters in common that distinguishes them from all other 
trees and plants. Such a group of species is called a genus. 
Families or Orders, in botanical language, are groups 
of genera that agree in certain particulars. Thus the sev- 
eral plants well-known as mallows, hollyhock, okra, and 
cotton, are representatives of as many different genera. 
They all agree in a number of points, especially as regards 
the structure of their fruit. They are accordingly group- 
ed together into a natural family or order, which differs 
from all others. , 
Classes, Series, and Classification.— Classes are groups 
* For a masterly statement of the facts and evidence bearing on these points, 
which are of the greatest importance to the agriculturist, see Darwin's works 
“On the Origin of Species,” and ‘‘On the Variation of Animals and Plants 
under Domestication.” 
