Let us suppose that we have laid on the table... [a] piece of glass... and let us homologize this glass to a whole order of plants or birds. Let us hit this glass a blow in such a manner as but to crack it up. The sectors circumscribed by cracks following the first blow may here be understood to represent families. Continuing, we may crack the glass into genera, species and subspecies to the point of finally having the upper right hand corner a piece about 4 inches square representing a sub-species.