There are transitional forms between the metals and non-metals; between chemical combinations and simple mixtures, between animals and plants, between phanerogams and cryptogams, and between mammals and birds [...]. The improbability may henceforth be taken for granted of finding in Nature a sharp cleavage between all that is masculine on the one side and all that is feminine on the other; or that any living being is so simple in this respect that it can be put wholly on one side, or wholly on the other, of the line.