Why are the classification systems changing every now and then?

Based on the sources provided, the specific reasons **why** classification systems change “every now and then” are not explicitly stated as an answer to this question, although the question itself is posed in the exercises section.

However, the sources do provide context that suggests why changes might be necessary or occur:

* It is **nearly impossible to study all the living organisms** presently existing, and new organisms are **continuously being identified**. As more organisms are discovered and described, the existing classification system might need adjustments to accommodate them logically.
* Classification is based on various characteristics, including external and internal structure, cell structure, development process, and ecological information. As our **knowledge about these characteristics improves** or new methods of studying them (though not mentioned in the source) become available, the basis for classification can evolve, leading to changes.
* Systematics, which includes classification, **takes into account evolutionary relationships** between organisms. As our understanding of the evolutionary history and relationships between different groups of organisms improves, the taxonomic arrangements based on these relationships might be revised.

In summary, while the source poses the question, it doesn’t directly list the reasons for changes. However, it describes a field where new discoveries are constant, the basis for classification is detailed characteristics and evolutionary relationships, and the goal is to facilitate the study of a vast diversity of life. These factors imply that as knowledge grows and our understanding deepens, classification systems would naturally be subject to revision.