There is a classification system, but it's a neat 'cubby-hole' approach to systems biology. The reality is that not everything has a clean parent-child relatio

There is a classification system, but it's a neat 'cubby-hole' approach to systems biology. The reality is that not everything has a clean parent-child relationship. For example, humans have the ABO blood group system. If you have A type blood you are not a 'subclass' of a human with no blood type. All ridiculous discussions of biology aside, having a class based hierarchy is a maintenance issue in comparison to designing classes through compositing. There are 2 ways to design a 'cheeseburger': a) meat sandwich is a super class of burger which is a super class cheeseburger b) meat sandwich which contains, bun, cheese, patty lettuce B is a more maintainable system and allows for better class flexibility in the design. It avoids having complex bugs where 'meat sandwich' changes some internals and breaks something down the hierarchy chain in some subtle way (for example in releasing a access to a critical resource).