Although global abnormalities in white matter and grey matter are not associated with cannabis abuse, reduced hippocampal volume is consistently found. Amygdalar abnormalities are sometimes reported, although findings are inconsistent.[105][106][107] Preliminary evidence suggests that this effect is largely mediated by THC, and that CBD may even have a protective effect.[108]
Millennia of selective breeding have resulted in varieties that display a wide range of traits; e.g. suited for a particular environments/latitudes, producing different ratios and compositions of terpenoids and cannabinoids (CBD, THC, CBG, CBC, CBN...etc.), fibre quality, oil/seed yield etc. Hemp grown for fiber is planted closely, resulting in tall, slender plants with long fibers.
One of the most significant developments for the North American hemp industry was investment in hemp products by Anita and Gordon Roddick, founders of The Body Shop, a well known international chain of hair and body care retailers. This was a rather courageous and principled move that required overcoming American legal obstacles related to THC content. The Body Shop now markets an impressive array of hemp nutraceutical cosmetics (Fig. 39), and this has given the industry considerable credibility. The Body Shop has reported gross sales of about a billion dollars annually, and that about 4% of sales in 2000 were hemp products.

The genus Cannabis was first classified using the "modern" system of taxonomic nomenclature by Carl Linnaeus in 1753, who devised the system still in use for the naming of species.[59] He considered the genus to be monotypic, having just a single species that he named Cannabis sativa L. (L. stands for Linnaeus, and indicates the authority who first named the species). Linnaeus was familiar with European hemp, which was widely cultivated at the time. In 1785, noted evolutionary biologist Jean-Baptiste de Lamarck published a description of a second species of Cannabis, which he named Cannabis indica Lam.[60] Lamarck based his description of the newly named species on plant specimens collected in India. He described C. indica as having poorer fiber quality than C. sativa, but greater utility as an inebriant. Additional Cannabis species were proposed in the 19th century, including strains from China and Vietnam (Indo-China) assigned the names Cannabis chinensis Delile, and Cannabis gigantea Delile ex Vilmorin.[61] However, many taxonomists found these putative species difficult to distinguish. In the early 20th century, the single-species concept was still widely accepted, except in the Soviet Union where Cannabis continued to be the subject of active taxonomic study. The name Cannabis indica was listed in various Pharmacopoeias, and was widely used to designate Cannabis suitable for the manufacture of medicinal preparations.[62]

The scientific debate regarding taxonomy has had little effect on the terminology in widespread use among cultivators and users of drug-type Cannabis. Cannabis aficionados recognize three distinct types based on such factors as morphology, native range, aroma, and subjective psychoactive characteristics. Sativa is the most widespread variety, which is usually tall, laxly branched, and found in warm lowland regions. Indica designates shorter, bushier plants adapted to cooler climates and highland environments. Ruderalis is the informal name for the short plants that grow wild in Europe and Central Asia.