(a) the place where they live.
(b) the kind of cells they are made of. Why?
-Suchismita ,Subject Matter Expert
The most basic classification depends on the place where they live. As if the classifying criteria will be morphological i.e. (physical) which is observed by naked eye.
Suppose if we classify animals which live in mountains and animals that live in tropical forests.
Such as polar bear, snow leopard, penguin , walrus all these animals belong in the polar region or royal bengal tiger , lion, monkey, deer, kangaroo and all others belong to the tropical region.
But if we want to classify organisms on their basic life structure then the kind of cells will be a better criterion. If we want to study an organism from where it originated from then we have to start from the basics.
Suppose starting from unicellular organisms (amoeba) to human beings classification can be done in very different ways. Such as amoeba is unicellular whereas it is multicellular (trillions of cells are present). Maybe some kinds of cells are similar, may be at a very primitive level and some are different.
But classifying them can as a group according to cell structure get some similarity i.e. the basic structure. Such as amoeba and protozoa both having primitive forms of nucleus. And other classification of animals such as monkeys and human beings both have well developed nuclei .
So, classifying on the basis of a cell is much more prominent that the place where they live. By this type of classification we can see the flow of evolution on the cellular basics.