ISOLATION 195 



sider the whole stock-in-trade, so to speak, of a species, not 

 one particular article. Take an illustration from the animal 

 world. The Grasshopper-warbler- (Locustella nxvia) hides her 

 nest with great skill. She sits close, so that it is difficult to 

 flush her. The thrush (Turdus musicus), on the other hand, 

 scarcely aims at concealment : few nests are easier to find than 

 hers. And yet thrushes are far more numerous than grasshopper 

 warblers. This is only one instance among thousands to show 

 that if we wish to understand how different species hold their 

 own, it is no use singling out one characteristic of each, and 

 comparing them in respect of that alone. We must try to 

 learn all their characteristics, , their whole life-histories, and the 

 conditions under which they live. This is, of course, only a 

 counsel of perfection, but if we make it our aim, we shall not 

 fall into the error of judging by .one peculiarity alone. In the 

 present case we can fall back upon the fact, proved by experi- 

 ments, that cross-fertilisation does increase the vigour of plants. 



It has now been shown that flowers require cross-fertilisation Summary 

 if the stock is to maintain its vigour, and that crossing is carried 

 on in very many cases through the agency of insects ; that inter- 

 sterility or comparative infertility between varieties has isolated 

 them so that the defective constancy of bees has not led to the 

 swamping of varieties before they were worthy to rank as 

 species ; that bees have a colour-sense, and that owing to this 

 they have fostered and accumulated variations in the direction 

 of increased brilliancy. Insects have been the gardeners that 

 have covered the earth with bright flowers. But they could not 

 have brought about this result, had not many new varieties 

 become at an early stage infertile with each other and with the 

 parent species. We have to thank the colour-sense of bees and 

 the infertility between varieties and between closely related 

 species, working together, for all the colours, forms and scents 

 of flowers. 



