Cannabis - Sativa, Indica and Ruderalis
Sativa, Indica and Ruderalis are all sub-species of the genus cannabis, and are all equally 'illegal' under UK law.
They can all inter-breed freely, and many 'pedigree' cultivars are indica/sativa hybrids. Authorities disagree about the number of species of plant which constitute the genus Cannabis. Although many authorities continue to class all varieties of the plant, including Hemp and Marijuana, as Cannabis sativa, it is widely accepted that there are three separate species or sub-species:
C.sativa being most widely cultivated in the Western World, was originally grown on an industrial scale for fibre, oil, and animal feedstuffs, is characterised by tall growth with few, widely-spaced, branches;
Cannabis indica, originating in south Asia, and also known historically as Indian Hemp, was cultivated for the drug content, with shorter bushy plants giving a much greater yield per unit height; Cannabis ruderalis is a hardier variety grown in the northern Himalayas and southern states of the former Soviet Union, having a more sparse "weedy" growth, and is rarely cultivated for the drug content .
Those who argue that all three are one species point to the fact that cross-breeding produces viable and fertile daughter plants. Technically, therefore, they can be so considered, in the same (albeit less extreme) manner as a Great Dane and a Chihuahua are technically the same species of dog.