HOW PLANTS LIVE AND WOKK. 343 



These reasons may be grouped under four general heads : — 



I. — It is perfectly obvious that of the innumerable fresh plants 

 which are constantly brought into existence, only a small number 

 can possibly reach maturity. The amount of plant-food on the 

 earth is probably no greater than it was a million years ago, and it 

 is extremely unlikely that there has been any marked increase in 

 the total sum of plant-life since that period. 



2. — The individuals which survive do so because they are in 

 some respects better fitted than their contemporaries to cope with 

 the difficulties and dangers by which they are constantly beset. 



3. — There is a tendency for every fresh plant to resemble its 

 parent or parents more than anything else. 



4. — Just as no man is exactly like his father, so no plant 

 resembles its parent or parents in every particular. 



Let us now apply these considerations to our Wallflower. 

 Wallflowers, like all other plants with gaily coloured corollas, have 

 come into existence comparatively recently. They are descended 

 from ancestors which had insignificant flowers. Now, suppose 

 that, ages ago, one or two of the plants from which Wallflowers 

 are descended happened to have blossoms which were slightly 

 coloured. It is clear that in the struggle for existence these 

 individuals would have a slight advantage over their more incon- 

 spicuous neighbours, for they would be more easily visible to such 

 insects as frequented plants for the sake of pollen. Other things 

 being equal, the chances of cross-fertilisation would be greater in 

 the case of these fortunate plants than in that of their less favoured 

 fellows. The new plants which resulted from such cross-fertilisa- 

 tion would not only be stronger and more vigorous on that very 

 account, but they would also inherit from their parents a tendency 

 to produce coloured blossoms. It would thus gradually come 

 about that only such plants of the stock as produced coloured 

 flowers would have any chance in the struggle for existence, which, 

 as I have said, is extremely keen. The amount of colouration at 

 this stage would, of course, be very small, but it would be quite 

 sufficient to turn the scale. The process would still go on, and 

 those flowers whose colouring was a little stronger than that of 

 their competitors would again be favoured, until ultimately a dis- 

 tinctly new species would be produced. 



