I had undertaken the study of the radiolaria as a kind of souvenir of my great master, Johannes Müller. He had loved to study these animals (of which only a few species were discovered for the first time in the year of my birth, 1834) in the last years of his life, and had in 1855 set up the special group of the rhizopods (protozoa). His last work, which appeared shortly after his death (1858), and contained a description of 50 species of radiolaria, went with me to the Mediterranean when I made my first long voyage in the summer of 1859. I was so fortunate as to discover about 150 new species of radiolaria at Messina, and based on these my first monograph of this very instructive class of protists (1862). I had no suspicion at that time that fifteen years afterwards the deep-sea finds of the famous _Challenger_ expedition would bring to light an incalculable wealth of these remarkable animals. In my second monograph on them (1887), I was able to describe more than 4,000 different species of radiolaria, and illustrate most of them on 140 plates. I have given a selection of the prettiest forms on ten plates of my _Art-forms in Nature_. I have not space here to go into the forms and vital movements of the radiolaria, of the general import of which my friend, Wilhelm Bölsche, has given a very attractive account in his various popular works. I must restrict myself to pointing out the general phenomena that bear upon our particular subject, the question of the mind. The pretty flinty skeletons of the radiolaria, which enclose and protect the soft unicellular body, are remarkable, not only for their extraordinary gracefulness and beauty, but also for the geometrical regularity and relative constancy of their forms. The 4,000 species of radiolaria are just as constant as the 4,000 known species of ants; and, as the Darwinian Jesuit, Father Wasmann, has convinced himself that the latter have all descended by transformation from a common stem-form, I have concluded on the same principles that the 4,000 species of radiolaria have developed from a primitive form in virtue of adaptation and heredity. This primitive form, the stem-radiolarian (_Actissa_) is a simple round cell, the soft living protoplasmic body of which is divided into two different parts, an inner central capsule (in the middle of which is the solid round nucleus) and an outer gelatinous envelope (_calymma_). From the outer surface of the latter, hundreds and thousands of fine plasmic threads radiate; these are mobile and sensitive processes of the living internal substance, the plasm (or protoplasm). These delicate microscopic threads, or pseudopodia, are the curious organs that effect the sensations (of touch), the locomotion (by pushing), and the orderly construction of the flinty house; at the same time, they maintain the nourishment of the unicellular body, by seizing infusoria, diatoms, and other protists, and drawing them within the plasmic body, where they