different species of organisms have different inherited characteristics & the size of an organism is an inherited characteristic ->  different species of organisms will have different inherited characteristics in size  different species of organisms will have different inherited characteristics in size & genes are able to determine the inherited characteristics of a living thing ->  different species of organisms have different genes determining the size  different species of organisms have different genes determining the size & tigers and household cats are different species -> genetic difference in tigers and cats determines the difference in size