272 Practical Plant Biology, 



considered to the common stock. Thus the relationships revealed 

 by classification are really genetic relationships and the different 

 species included in a genus are the terminations of different lines 

 of descent from a common ancestral stock. Evidently the applica- 

 tion of the theory cannot stop here. The possibility of grouping 

 genera into families and families in turn into orders forces us, on 

 the same principle, to admit common origins for these groups, and 

 we must regard the relationship connecting the living species of 

 organisms as comparable to a vast and most complex genealogical 

 tree, starting the pedigree with those forms of life not distin- 

 guished as belonging to either the vegetable or animal kingdom, 

 and branching, with the growth of complexity and differentiation, 

 into kingdoms, sub-kingdoms, divisions, classes, orders, families, 

 genera and species. Thus each species, fitting as it does into its 

 place among the outermost branches of this genealogical tree and 

 indicating by its structure its relationships to its neighbours, adds 

 evidence in favour of the theory of descent. 



(b) Argument from modified parts. Naturalists are familiar 

 with examples both in plants and animals of parts which, appearing 

 in two different species and presenting underlying and funda- 

 mental similarities, are yet strikingly modified in shape and struc- 

 ture.- For instance, while the leaves of many leguminous plants 

 are subdivided into small leaf-like pinnae, in others some of the 

 pinnae are represented by filiform tendrils, which by twining round 

 supports sustain the weight of the plant. Such tendrils have long 

 been described as modified leaves. The rational explanation of 

 such modified parts is, as the name implies, that the ancestors of 

 the plant in question were possessed of flat pinnae, like some of 

 their living descendants, while other descendants "have become 

 modified into tendril-bearing species. In the same way we find the 

 spines of Cacti represent modified leaves. Other plants furnish 

 examples of leaves being modified into floating organs, into traps 

 for the capture and digestion of animals, into organs for the ac- 

 cumulation of humus, etc. Leaves are not the only organs which 

 are subject to these modifications but each and every part furnishes 

 similar examples. In fact all the distinguishing characters between 

 allied species are modifications of ancestral parts and all emphasise 

 the reality of the origin of species by descent. 



(c) The argument from the existence of homologues is in reality 

 an extension of that based on modified parts. If each species 

 were the result of a special creative act there would be no reason 

 why there should be found in widely divergent species parts homo- 

 logous to one another. But if you regard the existing species, no 



