A. H. Trow 271 



The constitution of the pure types has not been adequately in- 

 vestigated, but already the results are of interest. The constitution 

 of the twelve already referred to is given below : — 



Praecox — rrCChhijyzzGGLL. 



Erectiis—rrGGhhXXYYZZggll. 



Multicaulis — rrcchhxxYYZZggLL. 



Latifolius — rrhhggll. 



Genevensis — rrhhGG. 



Erectus, radiatus — RRCGhh XXZZggll. 



Lanuginosus — RRHHyyGGLL. 



Cardiff Groundsel \ 



Burri/ Green Groundsel 



Horton Groundsel /■ rrHHGG. 



Gross Common Groundsel 



St Bride's Groundsel I 



Crosses are apparently possible between any pair of these. The 

 cross multicaulis x erectus, radiatus gives a hybrid of the constitution 



RrLlCcXx helerozygotic for four pairs of factors, and there should 



thus be at least 18 distinct types of plant in the F.^ generation, 

 recognisable by inspection, without further experiment. The cross 

 lanuginosus x multicaulis gives a hybrid of the constitution HhGgRrYy, 

 and there should be at least 54 recognisable types in the F„ generation. 

 Moreover several other factors affect the form and habit of the plants, 

 producing still further diversity of type. Consequeutly there must be 

 several hundreds of these trroundsel forms still unidentified, but never- 

 theless recognisable. Such facts shew how inadequate the older methods 

 of study are for the investigation not only of hybrids, but of critical 

 species also. 



After an investigation extending over six years, including the critical 

 examination of about 10,000 groundsel plants, I still often find it very 

 difficult to estimate, even provisionally, the constitution of a casual wild 

 plant. Yet the methods of genetics, diligently applied, obviously give 

 one the power to replace loose speculation and guesswork by irrefutable 

 inductions, and so to lay down a foundation upon which the evolutionist 

 and taxonomist can build with safety. 



April I'lth, 1912. 



19—2 



