“Essay on Classification.”  The larvae of barnacles and other more degraded parasitic crustacea are almost exactly like those of Crustacea in general.  The embryos of birds have a long tail containing almost or quite as many vertebrae as that of archaeopteryx.  But most of these never reach their full development but are absorbed into the pelvis, or into the “ploughshare” bone supporting the tail feathers.  Thus older forms may be said to have retained throughout life a condition only embryonic in their higher relatives.  And the natural classification gave the order not only of geological succession but also of stages of embryonic development.  Thus the system of classification improved continually, although more and more intermediate forms, like archaeopteryx, were discovered, and certain aberrant groups could find no permanent resting-place.

But why should the generalized comprehensive forms stand at the bottom rather than the top of the systematic arrangement of their classes?  Why should the system of classification coincide with the order of geologic occurrence, and this with the series of embryonic stages?  Above all, why should the embryos of bird and perch form their tails by such a roundabout method?  Why should the embryo of the bird have the tail of a lizard?  No one could give any satisfactory explanation, although the facts were undoubted.

Mr. Darwin’s theory was the one impulse needed to crystallize these disconnected facts into one comprehensible whole.  The connecting link was everywhere common descent, difference was due to the continual variation and divergence of their ancestors.  The classification, which all were seeking, was really the ancestral tree of the animal kingdom.  Forms more generalized should be placed lower down on the ancestral tree, and must have had an earlier geological occurrence because they represented more nearly the ancestors of the higher.  But this explains also the facts of embryonic development.

According to Mr. Darwin’s theory all the species of higher animals have developed from unicellular ancestors.  It had long been known that all higher forms start in life as single cells, egg and spermatozoon.  And these, fused in the process of fertilization, form still a single cell.  And when this single cell proceeds through successive embryonic stages to develop into an adult individual it naturally, through force of hereditary habit, so to speak, treads the same path which its ancestors followed from the unicellular condition to their present point of development.  Thus higher forms should be expected to show traces of their early ancestry in their embryonic life.  Older and lower adult forms should represent persistent embryonic stages of higher.  It could not well be otherwise.