Why are scientific theories modified, but seldom discarded? explain

Hello.

The original theories are based on a certain amount of truth. Now a days, theories are not accepted until there is a significant amount of evidence to back it up. If there should be new evidence that states otherwise, then the theory will be modified.  The reason that it is called a theory, is because it can't be proven yet. This means that it might still become accepted some day, but until that time, pieces of the theoretical puzzle will have to be put together gradually (modified) as new information comes in and knowledge is gained.

Have  a nice day

several tissues working together form an organ, and several organs together make up an organisms.

the group of cell that carries out special jobs are tissue.