A little later it secured a well-merited recognition in botany also, especially owing to the efforts of Matthias Schleiden of Jena, the distinguished student who provided biology with a new foundation in the "cell theory" (1838). But it was not until the middle of the nineteenth century that people generally recognised that the ovum of the plant or animal is itself only a simple cell, and that the later tissues and organs gradually develop from this "elementary organism" by a repeated cleavage of, and division of labour in, the cells. The most important step was then made of recognising that our human organism also develops from an ovum (first discovered by Baer in 1827), in virtue of the same laws, and that its embryonic development resembles that of the other mammals, especially that of the ape. Each of us was, at the beginning of his existence, a simple globule of protoplasm, surrounded by a membrane, about 1/120 of an inch in diameter, with a firmer nucleus inside it. These important embryological discoveries confirmed the rational conception of the human organism that had been attained much earlier by comparative anatomy: the conviction that the human frame is built in the same way, and develops similarly from a simple ovum, as the body of all other mammals. Even Linné had already (1735) given man a place in the mammal class in his famous _System of Nature_. Differently from these embryological facts, which can be directly observed, the phenomena of phylogeny (the development of species), which are needed to set the former in their true light, are usually outside the range of immediate observation. What was the origin of the countless species of animals and plants? How can we explain the remarkable relationships which unite similar species into genera and these into classes? Linné answers the question very simply with the belief in creation, relying on the generally accepted Mosaic narrative: "There are as many different species of animals and plants as there were different forms created by God in the beginning." The first scientific answer was given in 1809 by the great French scientist, Lamarck. He taught, in his suggestive _Philosophie Zoologique_, that the resemblances in form and structure of groups of species are due to real affinity, and that all organisms descend from a few very simple primitive forms (or, possibly, from a single one). These primitive forms were developed out of lifeless matter by spontaneous generation. The resemblances of related groups of species are explained by _inheritance_ from common stem-forms; their dissimilarities are due to _adaptation_ to different environments, and to variety in the action of the modifiable organs. The human race has arisen in the same way, by transformation of a series of mammal ancestors, the nearest of which are ape-like primates.