In this connection a point must be brought up that has played an important rôle in all discussion as to what limits can be set to a species. As a rule it is found that two distinct 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 possible to cross-fertilize two distinct species, although the offspring 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 invariably 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 inventions of man for purposes of classification. Having discovered that it is possible to arrange animals and plants in groups within groups, the question arises as to the meaning of this relation. Have these facts any other significance than that of a classification of geometric figures, or of crystals according to the relations of their axes, or of bodies as to whether they are solids, liquids, or gases, or even whether they are red, white, or blue? If we accept the transmutation view, we can offer an explanation of the grouping of living things. According to the transmutation theory, the grouping of living things is due to their common descent, and the greater or less extent to which the different forms have diverged from each other. It is the belief in this principle that makes the classification of the biologist appear to be of a different order from that in any other science; and it is this principle that appears to give us an insight into a large number of phenomena.