Figure 1.

The hierarchical quality of complexity during development. Organismal complexity and the interactions across nested levels of complexity. Two measures of complexity can be obtained. The first measure, horizontal complexity is assessed at a single level of biological organization (i.e., number of different cell types, or number of different organs). The second measure, vertical complexity refers to the numbers of hierarchically nested steps between the lowest rank of biological organization, of the system studied, and the highest. Red arrows show interactions between parts at a given level and between different nested levels. The diagrams represent animal cells, tissues, and organs by convenience; however, the same ideas are applicable to non‐animal organisms [Color figure can be viewed at wileyonlinelibrary.com]