p. 29
Chapter II

    Among living bodies the name affinity has been given to features of analogy or resemblance between two objects, that are compared in their totality, but with special stress on the most essential parts.  The closer and more extensive the resemblance, the greater the affinities.  They indicate a sort of kinship between the living bodies which exhibit them; and oblige us in our classification to place these bodies in a proximity proportional to their affinities.

    How great has been the progress of natural science since serious attention began to be given to affinities, and especially since their true underlying principles have been determined!

    Before this change, our botanical classifications were entirely at the mercy of arbitrary opinion, and of artificial systems of any author.  In the animal kingdom, the invertebrate animals comprising the larger part of all known animals were classified into the most heterogeneous groups, some under the name of insects, some under the name of worms; where the animals included are from the point of view of affinity widely different from one another.


p. 33
    We must then be guided everywhere by natural affinities in composing the groups which result by dividing each kingdom into classes, each class into orders, each order into sections or families, each family into genera, and each genus into different species if there is occasion for it.

    There is thorough justification for the belief that the complete series of beings making up a kingdom represents the actual order of nature, when it is classified with direct reference to affinities; but, as I have already pointed out, the different kinds of divisions which have to be set up in that series to help us distinguish objects with greater ease do not belong to nature at all. They are truly artificial although they exhibit natural portions of the actual order instituted by nature.