 The distinction between phenomena and mechanisms assumes specificity; i.e., there are specific phenomena (somite formation in vertebrates) or mechanisms (collinear Hox gene expression) in view when judging the relationship between source (model) and target. But animal development consists of a multitude of different processes that involve a host of different mechanisms. Therefore, another distinction operating in the representational criterion pertains to questions of specificity versus variety when selecting and using model organisms. A model might represent one type of target phenomena (differentiation or growth) or mechanism (cell signaling or cell cycling) but not others—specificity—or may do so better or worse with respect to particular types of phenomena or mechanisms. A model might represent several types of target phenomena and mechanisms simultaneously—variety—with variability in how each type is represented. Trade-offs exist with respect to how well different phenomena or mechanisms are co-instantiated in a model organism. Note that experimental organisms may be selected with respect to variety and specificity simultaneously, such as when a biologist working on a specific phenomenon intends to work on others using the same model in the future. They also may be selected with one or the other of these two aspects predominant. A model might be desirable if it has representational variety in both mechanisms and phenomena even if it is not the best representative for every specific mechanism or phenomenon. Conversely, a model organism might be desirable if it is the best representative for a specific mechanism despite being a very poor model for other phenomena or mechanisms. Variety is indicative of the “whole organism” being the  model.[17]  A further distinction can be introduced between “model organisms” and “experimental organisms” (Ankeny and Leonelli 2011) or “general model organisms” and “Krogh-principle model organisms” (Love 2010). General model organisms are selected and used with the variety aspect of the representational criterion preeminent; experimental or Krogh-principle model organisms are selected and used with specificity preeminent.