137 

 flower, crosses one with another." Climate is no doubt an 

 important factor. 



Now, everyone knows how vigorous are plants like 

 knot-weed, horse-weed, buttercups, thistle, plantains, etc. 

 They are world wide and it is equally true that they are self- 

 ferailized. Wallace says: -'That self-fertilization has some 

 great advantages is shown by the fact that it is usually the 

 species which have the smallest and least conspicuons flowers 

 tvhich have spread widely, while the large and showy 

 flowersd species of the same genera or families, which re- 

 quire insects to fertilize them have a much more limited dis- 

 tribution." 



It must be admitted that inter-crossing is not necessarily 

 good, but also that close inter-breeding is not necessarily 

 bad. Says vVallace: "The rabbits on the Island of Porto 

 Santo have originated from a single female. They have 

 lived there for 470 years and are still vigorous." But close 

 inter-breeding does produce bad results, especially if care in 

 selection is not used. The inter-breeding is not injurious if 

 there is a change of con.iitions and proper selection. 



Wallace concludes: "From a consideration of these 

 varied facts we conclude that an occasional disturbance of 

 the organic equilibrium is what is essential to keep up the 

 vigor and fertility of any organism, and that this disturb- 

 ance may be equally well produced, either by a cross between 

 individuals of somewhat different constitutions, or by occa- 

 sional slight changes in the condition of life." In plants of 

 wide dispersion there is a constant change of conditions; 

 these can exist for a long time without inter-crossing, but 

 in plants not so widely distributed a cross is needed to 

 keep up the fertility. Why the production of an enormous 

 quantity of pollen in case of the pine and other wind 



