and if it persisted might result in actual extermination. It is conceivable, therefore, that natural selection is capable of favouring stable and non-progressive types at the expense of the variable and possibly “progressive,” if such a term can be applied to species advancing towards extinction. Such a possible fixative action of natural selection is suggested by the fact that the appearance of mechanisms whereby the individual is protected from the direct action of natural selection seems to have led to an outburst of variation. In the multicellular animal the individual cells passing from under the direct pressure of natural selection become variable, and so capable of a very great specialization. In the gregarious unit the same thing happens, the individual member gaining freedom to vary and to become specialized without the risk that would have accompanied such an endowment in the solitary state.
Within the gregarious unit, then, natural selection in the strict sense is in abeyance, and the consequent freedom has allowed of a rich variety among the individual members. This variety provides the material from which