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.
4. This reduction of known biological phenomena to simpler terms, the lessening of the gap between inorganic and organic chemistry, and the formulation of the doctrine of the conservation of energy, have all prepared the way for a fourth step, the establishment of the inevitable theory of _generatio spontanea sive equivoca_, the spontaneous generation of the living, that is to say, the gradual evolution of the living from the not living. Since the earth, and with it the conditions under which alone life is possible, have had a beginning in time, life upon the earth must also have had a beginning. The assumption that the first living organisms may have come to the earth on meteorites simply shifts the problem a step farther back, for according to all current theories of the universe, if there are in any of the heavenly bodies conditions admitting of the presence of life, these conditions have arisen from others in which life was impossible. Therefore, since this suggestion is on the face of it a mere evasion of the difficulty, the theory of spontaneous generation naturally arose. There is something almost comical in the change in the attitude of the natural sciences to this theory. For centuries it was one of the beliefs of popular superstition, with its naïve way of regarding nature, that earthworms “developed” from damp soil, and vermin from shavings, and in general that the living arose from the non-living. On the other hand it was one of the characteristics and axioms of scientific thought to reject this naïve _generatio equivoca_, and to hold fast to the proposition, _omne vivum ex ovo_, or, at least, _omne vivum ex vivo_. And it was regarded as one of the triumphs of modern science when, about the middle of the last century, Pasteur gave definiteness to this doctrine, and when through him, through Virchow, and indeed the whole younger generation of naturalists, the proposition was modified, on the basis of the newly discovered cell-theory, to _omnis cellula ex cellula_. But a short time after Pasteur’s discoveries, the ideas of Darwinism and the theory of evolution gained widespread acceptance. And now it appeared that, in rejecting the theory of _generatio equivoca_, naturalists had, so to speak, sawn off the branch on which they desired to sit, and thus many, like Haeckel, became enthusiastic converts to the theory which natural science had previously rejected.