Fig.1. Medieval engraving of a scala naturae showing the "ladder" concept. The words on the steps read: rocks, flame (as a chemical reaction), plants, beasts, humans, heaven, angels, god.
The scala naturae or great chain of being placed all natural objects in a supposedly divine order and can be traced back to the ancient Greek Aristotelian philosophy. A common scala naturae in the 18th century started with less complex objects, like the (supposed) elements air, water, earth, to proceed towards metals, salts, rocks, corals (as half plants, half rock), lichens, higher plants, animals and finally Homo sapiens (just outclassed on the highest steps of the scala naturae by angels).
Until the 19th century also science reflected this chain, as it was divided in physics, chemistry and natural history, the latter including the study of animal, plants and rocks. Still in the various disciplines a certain order, from the inanimated to animated, is present. Around 1800 for the first time it was suggested that the science of “biology”, or the philosophy of life forms, should study the laws that rule and circumstances that enable life as we know it and, more important, be distinguished from geology as the study of earth and its lifeless matter. However the idea didn´t at first attract much interest and still in 1842 German botanist Matthias Jacob considered the proposed distinction between the animated an inanimated world an absurdity, as there could be found a gradual transition between every single object. However Jacob was among the last opponents, as indeed biology had started to become an own, distinct scientific discipline.
However still some interdisciplinary ideas survived. Like in crystallography, many biologists believed at the time that there exists, like for crystals, a smallest possible unit of life, a physiological unit or like Darwin proposed in 1867, a “gemmules”. Ernst Haeckel even named this supposed smallest entity (even smaller as a cell, at the time the smallest observable organic structure) “Kristalloid” in 1876. Like a crystal forms and can grow by putting together the basic unit cells, an organism was composed of smallest units, each possessing the “lifeforce” and so giving life to the entire being.
Only at the beginning of the 20th century and discovery of cellular organelles and DNA the idea of such "living basic units" was abandoned.