An immune system of enormous complexity is present in all vertebrate animals. When we place a population of lymphocytes from such an animal in appropriate tissue culture fluid, and when we add an antigen, the lymphocytes will produce specific antibody molecules, in the absense of any nerve cells. I find it astonishing that the immune system embodies a degree of complexity which suggests some more or less superficial though striking analogies with human language, and that this cognitive system has evolved and functions without assistance of the brain.

In the study of this membrane [the retina] I for the first time felt my faith in Darwinism (hypothesis of natural selection) weakened, being amazed and confounded by the supreme constructive ingenuity revealed not only in the retina and in the dioptric apparatus of the vertebrates but even in the meanest insect eye. ... I felt more profoundly than in any other subject of study the shuddering sensation of the unfathomable mystery of life.

The occurrence of an internal skeleton, in definite relations to the other organ systems, and the articulation of the body into homologous segments, are points in the general organization of Vertebrates to which especial weight must be given. This metameric structure is more or less definitely expressed in most of the organs, and as it extends to the axial skeleton, the latter also gradually articulates into separate segments, the vertebrae. The latter, however, must be regarded only as the partial expression of a general articulation of the body which is all the more important in consequence of its appearing prior to the articulation of the originally inarticulate axial skeleton. Hence this general articulation may be considered as a primitive vertebral structure, to which the articulation of the axial skeleton is related as a secondary process of the same sort.

Vertebrate development consists in the formation, in the median plane, of four leaflets two of which are above the axis and two below. During this evolution the germ subdivides in layers, and this has the effect of dividing the primordial tubes into secondary masses. The latter, included in the other masses, are the fundamental organs with the faculty of forming all the other organs.