Lntroductory. 19 
There would also be copies of the same work in different 
bindings which would correspond to varieties. 
22. Scientific Names are given to plants and animals 
because the common names by which they are known are so 
often local. For example, quack grass, one of our common 
troublesome weeds, is known by at least seven different com- 
mon names in this country alone, and yet, in a given locality 
it is often known by only one name. Its scientific name, 
however, Agropyrum repens (a-gro-py’-rum re’-pens), is the 
same in all languages and countries, Scientific names are 
usually Latin and consist of two words. The first word is 
the name of the genus to which the plant or animal belongs, 
and is called the generic (ge-ner’-ic) name; the second word 
designates the species, and is called the specific (spe-cif’-ic) 
name. For example, Pyrus malus (py’-rus ma’-lus) is the 
scientific name of the common apple, Pyrus being the genus 
to which the apple belongs, and malus designating which 
species of the genus is meant. 
23. Crosses and Hybrids (hy’-brids). We have seen 
(17), that in sexual reproduction, a union of male and female 
cells is almost always essential. When these cells proceed 
from two individuals of different varieties (21), the offspring 
is called a cross; when they proceed from individuals of differ- 
ent species, it is called a hybrid. Hybrids are possible only 
between closely allied species and are often incapable of re- 
production, in which case they are said to be sterile. The 
mule, which is a hybrid between the horse and the ass, is a 
familar example of a sterile hybrid. Sterile hybrids are not 
uncommon in plants. A hybrid that is capable of reproduc- 
tion is called a fertile hybrid. 
