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 r61e 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, i.e. 

 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. Even 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, inter 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. 



