Tribe 

 Genus 

 Section 

 Species 

 Sub-species 

 Race 

 Variety. 



The main object of classification is to enable us to rapidly 

 become acquainted with the principal groups of plants 

 indigenous in various countries, and it must be remembered 

 that the above definitions refer to groups of plants as they 

 exist growing wild in a state of nature, and that there are many 

 plants which, in the garden, breed true and give the impression 

 of being constant forms and of constituting good species which 

 are not found as wild species in a natural state, owing to 

 their being unable to survive in the struggle for existence 

 or to other causes. Hence, if terms similar to those given 

 above are used for analogous groups of cultivated plants, it 

 should invariably be stated that plants under cultivation 

 are referred to. 



110. Proof that a particular group Practical 



of individuals has descended from another group is not in Determina- 

 itself sufficient reason for combining them together as one ^ of the 

 species, for, in the course of time, the intermediate forms which Groups, 

 once united the two groups may have disappeared, causing 

 the two groups to occur in nature as distinct species separ- 

 ated by well-marked and constant differences. 



A species is a group of plants which actually exists in 

 nature, the recognition and correct definition of which are 

 independent of the way in which the group originated. 



With a few exceptions, the individuals of one and the 

 same species cross readily and produce fertile offspring while 

 individuals belonging to distinct species very frequently do 

 not do so. At the same time fertility cannot be accepted as 

 an infallible criterion of species, for illegitimate unions between 

 the different forms of flowers which occur in one and the same 

 species, in plants with dimorphic and trimorphic flowers, 

 produce very little fertile seed and the plants raised from such 

 seed are sterile inter se, just as is frequently the case with 

 the illegitimate unions between distinct species. The male and 

 female forms of some organisms differ widely from each other 

 in many important characters, while an organism may also 

 exhibit an entirely different appearance at different periods 



