The Theory of Evolution 33 
reached the limits of classification, the species being the 
unit. This, however, is far from being the case, for, in 
many species we find smaller groups, often confined to 
special localities. These groups are called varieties. 
In some cases it appears, especially in plants, these 
smaller groups of varieties resemble in many ways the 
groups of species in other forms, since they breed’ true to 
their kind, even under changed conditions. They have been 
recognized as “smaller species” by a number of botanists. 
In this connection a point must be brought up that has 
played an important réle in all discussion as to what limits 
can be set to a species. As a rule it is found that two dis- 
tinct species cannot be made to cross with each other, 2.2. 
the eggs of an individual of one species cannot be fertilized 
by spermatozoa derived from individuals of another species; 
or, at least, if fertilization takes place the embryo does not 
develop. In some cases, however, it has been found possi- 
ble to cross-fertilize two distinct species, although the off- 
spring is itself more or less infertile. ven this distinction, 
however, does not hold absolutely, for, in a few cases, the 
offspring of the cross is fertile. It cannot be maintained, 
therefore, that this test of infertility between species invari- 
ably holds, although in a negative sense the test may apply, 
for if two different forms are infertile, zzter se, the result 
shows that they are distinct species. If they cross they may 
or may not be good species, and some other test must be 
used to decide their relation. 
We should always keep in mind the fact that the individual 
is the only reality with which we have to deal, and that the 
arrangement of these into species, genera, families, etc., is 
only a scheme invented by man for purposes of classification. 
Thus there is no such thing in nature as a species, except as 
a concept of a group of forms more or less alike. In nature 
there are no genera, families, orders, etc. These are inven- 
tions of man for purposes of classification. 
D 
