that it places their organization below some more highly developed being built on a lower plan.  It is a poor comparison, because everything that God has made is perfect of its kind and in its place, though relatively lower or higher; yet it is only by comparison of what is after all akin,—­of mind with mind,—­even though so far apart as the works of the divine and the human reason, that we may arrive at some idea, however dim, of the mental operations of the Creative Intellect.

It is, then, in their whole bulk that any one of these groups is above any other.  We may represent the relative positions of the classes by a diagram in which each successive class in every type starts at a lower point than that at which the preceding class closes.  Taking the Polyps as the lowest class of Radiates, for instance, its highest animals rise above the lowest members of the Acalephs, but then the higher members of the class of Acalephs reach a point far above any of the Polyps,—­and so on.

RADIATES.          MOLLUSKS.          ARTICULATES.      VERTEBRATES.
|                 |                  |                 |
|                 |                  |               | |
| |               | |                | |               | Mammalia.
| |               | |                | |             | |
| | Echinoderms.  | | Cephalopoda.   | | Insects.    | | Birds.
| |               | |                | |             | |
| Acalephs.       | Gasteropoda.     | Crustacea.    | Reptiles.
|                 |                  |               |
Polyps.            Acephala.           Worms           Fishes.

If this view be correct, it sets aside the possibility of any uninterrupted series based on absolute superiority or inferiority of structure, on which so much ingenuity and intellectual power have been wasted.

But it is not merely upon the structural relations established between these groups by anatomical features in the adult that we must decide this question.  We must examine it also from the embryological point of view.  Every animal in its growth undergoes a succession of changes:  is there anything in these changes implying a transition of one type into another?  Baer has given us the answer to this question.  He has shown that there are four distinct modes of development, as well as four plans of structure; and though we have seen that higher animals of one class pass through phases of growth in which they transiently resemble lower animals of the same class, yet each one of these four modes of development is confined within the limits of the type, and a Vertebrate never resembles, at any stage of its growth, anything but a Vertebrate, or an Articulate anything but an Articulate, or a Mollusk anything but a Mollusk, or a Radiate anything but a Radiate.