344 APPENDIX 



by cultivation the origination of new varieties is favoured, and that 

 by man's labour many varieties are acquired which, under natural 

 conditions, would be lost; but nothing justifies the assumption 

 that the tendency to the formation of varieties is so extraordinarily 

 increased that the species speedily lose all stability, and their 

 offspring diverge into an endless series of extremely variable forms. 

 Were the change in the conditions the sole cause of variability we 

 might expect that those cultivated plants which are grown for 

 centuries under almost identical conditions would again attain 

 constancy. That, as is well known, is not the case, since it is pre- 

 cisely under such circumstances that not only the most varied but 

 also the most variable forms are found. It is only the Leguminosae, 

 like Pisum, Phaseolus,^ Lens, whose organs of fertilisation are pro- 

 tected by the keel, which constitute a noteworthy exception. Even 

 here there have arisen numerous varieties during a cultural period 

 of more than 1000 years under most various conditions; these 

 maintain, however, under unchanging environments a stability as 

 great as that of species growing wild. 



It is more than probable that as regards the variability of culti- 

 vated plants there exists a factor which so far has received little 

 attention. Various experiments force us to the conclusion that our 

 cultivated plants, with few exceptions, are members of various 

 hybrid series, whose further development in conformity with law is 

 varied and interrupted by frequent crossings inter se. The circum- 

 stance must not be overlooked that cultivated plants are mostly 

 grown in great numbers and close together, affording the most 

 favourable conditions for reciprocal fertilisation between the varie- 

 ties present and the species itself. The probability of this is sup- 

 ported by the fact that among the great array of variable forms 

 solitary examples are always found, which in one character or 

 another remain constant, if only foreign influence be carefully 

 excluded. These forms behave precisely as do those which are 

 known to be members of the compound hybrid series. Also with 

 the most susceptible of all characters, that of colour, it cannot 

 escape the careful observer that in the separate forms the inclina- 

 tion to vary is displayed in very different degrees. Among plants 

 which arise from one spontaneous fertilisation there are often some 

 whose offspring vary widely in the constitution and arrangement 

 of the colours, while that of others shows little deviation, and 



^ [Phaseolus nevertheless is insect-fertilised.] 



