"some characteristics are more fundamental then other organisum "why?
Some characters are considered to be basic or fundamental characteristics since they decide the broadest divisions among living organisms. They are independent of any other characteristics in their effects on the form and function of the organism. The characteristics in the next level would be dependent on the previous one and would decide the variety in the next level. Thus we can build up a whole hierarchy of mutually related characteristics to be used for classification.
For example - Presence or absence of nucleus is the basic/fundamental characteristic of classification. This would have an effect on every aspect of cell design. An eukaryotic cell has membrane-bound organelles, including a nucleus, which allow cellular processes to be carried out efficiently in isolation from each other. Therefore, organisms which do not have a clearly demarcated nucleus and other organelles would need to have their biochemical pathways organised in very different ways. Further, nucleated cells would have the capacity to participate in making a multicellular organism because they can take up specialised functions. Therefore, this is a basic characteristic of classification.
Other fundamental characteristics are:
i) Unicellular versus multicellular organisation of body
ii) Ability to produce ones own food versus having to get food from outside
iii) Levels of organisation