Thus, for instance, it is shown that the egg of the sea-urchin may be “stimulated” to development, not exclusively by the fertilising sperm, but even by a simple chemical agent, or that spermatozoids which are seeking the ovum to be fertilised may be attracted by malic acid. These are “reductions” of the higher phenomena of life to the terms of a lower and simpler process of “stimulus,” that is to say, to chemotropism in the second case and something analogous in the first. A further reduction would be to show that the movement of the spermatozoids towards the malic acid is not a “vitalistic” act, much less a psychically conditioned one, (that is, conditioned by “taste,” “sensation,” and the voluntary or instinctive impulse liberated thereby), but is a chemo-physical process, although perhaps an exceedingly complex one. It would be another “reduction” of this second kind, if, for instance, the well-known effect of light on plants, which makes them turn their leaves towards it (heliotropism), could be shown to be due to more rapid growth of the leaf on the shaded side, which would lift up the leaf and cause it to turn, or to an increase of turgescence on the shaded side, and if it could be shown that the increase in either case was a simple and obvious physical process, the necessary consequence of the decreased amount of light. It is obvious, and it is also thoroughly justifiable, that all attempts along these lines of interpretation should be undertaken in the first place in connection with the simplest and lowest forms of life. It is in the investigation of the “Protists,” the study of the vital phenomena of the microscopically minute unicellular organisms, that attempts of this kind have been most frequently made. And they follow the course we have just indicated; the “apparently” vitalistic and psychical behaviour of unicellulars (impulse, will, spontaneous movement, selecting and experimenting) is interpreted in terms of reflex processes and the “irritability” of the cell, and these again are traced back, like all stimulus-processes, to the subtle mechanics of the atoms. Spontaneous Generation.