Note 21 — Chapter IV · SPECIAL PROOFS, TAKEN FROM THE GENERAL LAWS OF ANIMAL STRUCTURE, OR FROM COMPARISON IN THE TAXONOMIC SERIES. General Principles.

Undoubtedly the true principle on which primary groups ought to be made is, identity of general plan of structure, or traceableness of homology throughout. For these groups are the great primary branches of the tree of life, and classification ought to represent degrees of genetic relationship. This was Agassiz’s principle, although he did not admit the genetic relation. This principle has been, it seems to us, too much neglected by later systematists.