sterility barriers may not exist within the genus, specifi- 
cally in wild populations which have not yet been ex- 
amined for this character. They may, indeed, show vary- 
ing degrees of reproductive isolation. 
Reproductive isolation can, of course, occur by means 
other than sterility barriers. It is well known that, in 
certain genera of plants, as in some animals, ‘‘acceptable’’ 
species exist where there are few or no sterility barriers 
present. The examples are many. These species have 
evolved with other types of isolating mechanisms, that 
are either mechanical, ethological or geographical. 
The significant phenomenon in Cannabis is that the 
combinations of morphological and anatomical (and pos- 
sibly also chemical) characters have maintained their in. 
tegrity, in spite of hybridization. The maintenance of 
these combinations of characters is a better indication of 
these reproductive barriers than that resulting from ex- 
perimentation with cultivated strains of doubtful origin. 
It is, furthermore, well recognized that species con- 
cepts must necessarily vary from one genus to another 
and from one family to another, dependent on the genetic 
peculiarities of the group under consideration. With the 
very different genetic backgrounds in different families, 
genera, etc., it is not at all surprising that the patterns 
of variation in these sundry groups may be quite differ- 
ent. There is not the equivalence of units amongst fami- 
lies of plants in the same sense of elements in chemistry. 
At one time, it was hoped that the species might be so 
rigorously defined that it would serve as the unit of evo- 
lution. Taxonomy has come a long way, however, since 
this belief, and taxonomists now hold that the popula- 
tion is the evolutionary unit, the biologically significant 
unit in plants. 
Plants were not made to be catalogued and classified. 
‘They can never easily and with complete satisfaction be 
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