Charles Darwin's evolutionary biology originated from exploring the macroworld: collecting and comparing species around the world. The idea of symbiogenesis stems from observing the microworld: the study of species (bacteria) and cells through the microscope.
While studying cells, biologists Lynn Margulis became to believe that chloroplasts in plant cells as well as mitochondria in all eukaryote cells were bacteria. Cells were engulfing each other, so she argued, and extended this assumption to cilia, flagella and centrioles. Lichens were hypothesized and finally became known to be multiple organisms: a symbiosis between an algae and fungus. Merezhkovsky “extrapolated” and speculated about trees: the forest a world of ancient cyanobacteria held up by trees in every leaf.
Merezhkowsky ended his life by suicide in 1921. Besides his interest in early evolution, he had troubling views, including his eugenics and racist writings . In contrast, Lynn Margulis' ideas and research is merging, in parts, towards acceptance and into mainstream biology within the rRNA-supported tree-of-life framework. Rob R. Dunn writes (page 163 in ):
If Margulis were right that our mitochondria had once been free-living microbes, their rRNA would be more similar to that of other microbes than it is to the rRNA in our nucleus. And it was. Here was almost unassailable support for Margulis. The mitochondria and even chloroplasts could be mapped onto [Carl] Woese's tree of life, as easily as if they were still free-ling microbes living in the ground.
Keywords: cytology, theoretical biology, molecular biology, ribosomal ribonucleic acid (rRNA), theory of evolution, beyond Charles Darwin, philosophy, symbiosis
References and further reading
 Rob R. Dunn: Every Living Thing. First Edition. HarperCollins Publishers, New York, 2009; pages 138 and 144.
 J. Sapp, F. Carrapiço and M. Zolotonosov: Symbiogenesis: the hidden face of Constantin Merezhkowsky [www.ncbi.nlm.nih.gov/pubmed/15045832].
 San Jose STS: Dr. Lynn Margulis [www.isepp.org/Pages/San Jose 04-05/MargulisSaganSJ.html].