The statement that the earliest progenitors of the human race were simple cells of this kind, and led an independent unicellular life like the amœba, has not only been ridiculed as the dream of a natural philosopher, but also been violently censured in theological journals as “shameful and immoral.” But, as I observed in my essay _On the Origin and Ancestral Tree of the Human Race_ in 1870, this offended piety must equally protest against the “shameful and immoral” fact that each human individual is developed from a simple ovum, and that this human ovum is indistinguishable from those of the other mammals, and in its earliest stage is like a naked amœba. We can show this to be a fact any day with the microscope, and it is little use to close one’s eyes to “immoral” facts of this kind. It is as indisputable as the momentous conclusions we draw from it and as the vertebrate character of man (see Chap. XI). Fig.19 Blood-cells that eat, or phagocytes, from a naked sea-snail. Fig. 19—Blood-cells that eat, or phagocytes, from a naked sea-snail (_Thetis_), greatly magnified. I was the first to observe in the blood-cells of this snail the important fact that “the blood-cells of the invertebrates are unprotected pieces of plasm, and take in food, by means of their peculiar movements, like the amœbæ.” I had (in Naples, on May 10th, 1859) injected into the blood-vessels of one of these snails an infusion of water and ground indigo, and was greatly astonished to find the blood-cells themselves more or less filled with the particles of indigo after a few hours. After repeated injections I succeeded in “observing the very entrance of the coloured particles in the blood-cells, which took place just in the same way as with the amœba.” I have given further particulars about this in my _Monograph on the Radiolaria._ We now see very clearly how extremely important the cell theory has been for our whole conception of organic nature. “Man’s place in nature” is settled beyond question by it. Apart from the cell theory, man is an insoluble enigma to us. Hence philosophers, and especially physiologists, should be thoroughly conversant with it. The soul of man can only be really understood in the light of the cell-soul, and we have the simplest form of this in the amœba. Only those who are acquainted with the simple psychic functions of the unicellular organisms and their gradual evolution in the series of lower animals can understand how the elaborate mind of the higher vertebrates, and especially of man, was gradually evolved from them. The academic psychologists who lack this zoological equipment are unable to do so.