306 Plant Life and Evolution 



give the plant a better chance in the struggle for 

 existence. It does not necessarily follow that the 

 more robust members of a lot of seedlings may be 

 the ones that finally survive, as accidents may occur ; 

 they may be ravaged by insects or birds, and indeed 

 might be preferred by these to their less vigorous 

 competitors. But taking all things into account, it 

 is reasonable to suppose that the more vigorous in- 

 dividuals will in the end get the advantage of their 

 weaker rivals, and leave a larger number of off- 

 spring to transmit their more robust constitution. 



Continuous and Discontinuous Variation. Biol- 

 ogists recognize two main forms of variations, small 

 or continuous variations forming a practically un- 

 broken series, connecting the extremes within the 

 species; and a second kind, the so-called discon- 

 tinuous variation, where there is the sudden ap- 

 pearance of a character without intermediate 

 ones between it and the type. Under the latter 

 head are included the " sports " of the gardener, and 

 the " mutations " of De Vries and his followers. 



Fluctuating variations may perhaps best be 

 studied in cultivated plants, especially those which 

 are known to be true species, where variation is not 

 due to hybridization. A single example will suffice 

 to illustrate this point. I have recently had in my 

 garden a large number of plants of Cosmos, a fa- 

 vorite autumn flower in California, and a good 

 botanical species. The plants were of three varie- 

 ties, white, pink, and crimson, and the variations 



