198 THE HAUNTS OF LIFE 
the possible seeds are to become real seeds 
which will sprout. And even when the pollen 
can pass from the stamens of the flower to the 
pistil of the same flower (self-pollination), 
the results are not usually so good as when 
the pollen is carried by insects (or by the 
wind) from one blossom to another. When 
there is cross-fertilisation the yield of seeds is 
better. And the plants so produced tend to 
be more variable, which will be a good thing 
if it is useful for the plant to change. 
The Third Great Invasion.—It was about 
the end of the time known as the Devonian 
or Old Red Sandstone that amphibians made 
their appearance, and, in the next age, the 
Carboniferous, when the coal measures were 
laid down, they had their golden age. These 
early amphibians, ancestors of our frogs and 
toads, newts and salamanders, were the ad- 
vance guard of the third great invasion, which 
eventually led to the appearance of reptiles, 
birds, and mammals. This third invasion 
meant the opening up of many new possibili- 
ties for animals, and, in the long run, it led 
to man. 
It is interesting to notice some of the new’ 
things that began with amphibians, the ad- 
