In spite of this progress in knowledge the belief in "spontaneous generation" of such excessively minute organisms as the bacteria and yeasts was general until Theodore Schwann in 1836 performed with them just the same experiment as Redi had performed with blow-flies in 1668. He showed that if a putrescible liquid (for instance, soup) were boiled in a retort so as to destroy all germs, and then the open neck of the retort was kept heated in a flame, so that no floating germs could enter alive, the soup did not putrefy, and no bacteria or other organisms appeared in it. The old notions, nevertheless, survive to this day. Peasants, fisher-folk, and even uneducated wealthy countrymen cling to them with the confidence arising from profound ignorance. And occasionally a man of some scientific training and knowledge astonishes the world by a futile attempt to show that the old fancies were true in regard, at any rate, to the lowest microscopic forms of life. But these are but the echoes of the past; we do not believe nowadays in "spontaneous generation," nor in sudden transformations of lower into higher forms of life. The doctrine, "_omne vivum e vivo_"--every living thing (in the present condition of our earth) is born from a living thing--is now held by scientific investigators as a reasonable generalisation of experience. On the other hand, Harvey's dictum, "Every living thing comes from an egg," is only true in a limited sense, namely, that whilst the individual among most larger animals and plants is always traceable to an egg-cell detached from a parental individual of a like kind of species, there are whole groups and series of lower animals and most plants in which the individual born or "developed" from an egg-cell does not proceed when grown to full size to reproduce in turn by eggs and fertilising sperms, but divides into two or more individuals or gives off detached buds or reproductive bulbs, which become separate individuals, and only after these and several successive generations of individuals have been thus produced "asexually," by fission or by budding, does a generation appear which produces true egg-cells and sperm-cells and reproduces by their means. Thus it is true that the individuals "budded off" or separated by fission from an asexual parent can be ultimately traced through one or more generations of previous asexual parents to an egg-cell produced and fertilised in the regular way, and with this important modification Harvey's dictum is justified. These facts and the wonderful histories of the animals and plants in which egg-and-sperm-producing generations "alternate" with generations which multiply by fission and budding have only been worked out in detail and by the aid of the microscope during the great century of scientific discovery which lies just behind us. Often the two generations, reproducing, the one by fission, the other by egg and sperm-cells, are alike in appearance, but often they are very