In his 2005 book, Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom, Sean Carroll summarizes how he understands what biological innovations needed to occur to explain developmental changes. Key for Carroll was the recognition that development occurred as a “logic of making a series of initially similar modules and then making them different from one another.” (34). Key to this realization is that bodies varied twice in the evolutionary and developmental logic. The first variation occurs at the level of segmentation of each animal. Animal bodies can be viewed as composed of a series of varied body segments, where each segment is similar to but not identical to the next segment. In the fruit fly, for instance, one segment makes wings and the next segment closer to the tail makes halteres. This is "the logic of making a series of initially similar modules". The second crucial variation, making the modules "different from one another" was that each of these segments could vary as well, despite there placement in the organism. This would explain the development of homeo-mutants, such as the four winged fly. Depending on some form of mutation the segment that was supposed to be an abdominal segment could be transformed into a thoracic segment.
This remarkable view of animal development also explains how differences and similarities occur between species. Imagine now that this basic module was not just for a specific species of animal, but for all types of animals. This would suggest an even third level of variation, where segments varied from animal to animal. Now all creatures could be explained through the variation of segmental construction. Morphological changes between species, such as the changes between the human arms and a bird wings are variations that occur in how the same materials and processes are organized. This is the importance of gothic moments for species development, these are the moments during development when significant and irreversible changes occur allowing for shared molecules to be expressed in novel ways.