condition of an animal cannot be its highest condition,—­it does not pass from a more perfect to a less perfect state of existence.  The history of its growth is, on the contrary, the history of its progress in development; and therefore, when we find that the first stage of growth in the Winged Insect transiently represents a structural character that is permanent in the lowest order of its class, that its second stage of growth transiently represents a structural character that is permanent in the second order of its class, and that only in the last stage of its existence does the Winged Insect attain its complete and perfect condition, we may fairly infer that this division of the class of Insects into a gradation of orders placing Centipedes lowest, Spiders next, and Winged Insects highest, is true to Nature.

This is not the only instance in which the embryological evidence confirms perfectly the anatomical evidence on which orders have been distinguished, and I believe that Embryology will give us the true standard by which to test the accuracy of our ordinal groups.  In the class of Crustacea, for instance, the Crabs have been placed above the Lobsters by some naturalists, in consequence of certain anatomical features; but there may easily be a difference of individual opinion as to the relative value of these features.  When we find, however, that the Crab, while undergoing its changes in the egg, passes through a stage in which it resembles the Lobster much more than it does its own adult condition, we cannot doubt that its earlier state is its lower one, and that the organization of the Lobster is not as high in the class of Crustacea as that of the Crab.  While using illustrations of this kind, however, I must guard against misinterpretation.  These embryological changes are never the passing of one kind of animal into another kind of animal:  the Crab is none the less a Crab during that period of its development in which it resembles a Lobster; it simply passes, in the natural course of its growth, through a phase of existence which is permanent in the Lobster, but transient in the Crab.  Such facts should stimulate all our young students to embryological investigation as a most important branch of study in the present state of our science.

But while there is this structural gradation among orders, establishing a relative rank between them, are classes and branches also linked together as a connected chain?  That such a chain exists throughout the Animal Kingdom has long been a favorite idea, not only among naturalists, but also in the popular mind.  Lamarck was one of the greatest teachers of this doctrine.  He held not only that branches and classes were connected in a direct gradation, but that within each class there was a regular series of orders, families, genera, and species, forming a continuous chain from the lowest animals to the highest, and that the whole had been a gradual development of higher out of lower forms.  I have already