It does not require any special study to see that there are certain groups of animals and of plants that are more like each other than they are like the members of any other group. It is obvious to every one that the group known as mammals has a combination of characters not found in any other group; such, for instance, as a covering of hair, mammary glands that furnish milk to the young, and a number of other less distinctive features. These and other common characteristics lead us to put the mammals into a single class. The birds, again, have certain common characters such as feathers, a beak without teeth, the development of a shell around the egg, etc., and on account of these resemblances we put them into another class. Everywhere in the animal and plant kingdoms we find large groups of similar forms, such as the butterflies, the beetles, the annelidan worms, the corals, the snails, the starfishes, etc. Within each of these groups we find smaller groups, in each of which there are again forms more like each other than like those of other groups. We may call these smaller groups families. Within the families we find smaller groups, that are more like each other than like any other groups in the same family, and these we put into genera. Within the genus we find smaller groups following the same rule, and these are the species. Here we seem to have reached a limit in many cases, for we do not always find within the species groups of individuals more like each other than like other groups. Although we find certain differences between the individuals of a species, yet the differences are often inconstant in the sense that amongst the descendants of any individual there may appear any one of the other variations. If this were the whole truth, it would seem that we had here 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.