Question: Why is it sometimes a difficulty to classify organisms?

Hayley Clissold answered on 25 Jun 2015:

Sometimes it can be difficult to classify organisms, because some organisms may seem to fall into multiple categories. For example, there is a single-celled organism called Euglena, which has characteristics of different groups – it contains chloroplasts (like plants), it doesn’t have a cell wall (like animals), and it has flagella (like many bacteria).