<P> "It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigour and fertility over the self - fertilised offspring from one of the same parents . And this fact is amply sufficient to account for the development of the sexual elements, that is, for the genesis of the two sexes ." </P> <P> In addition, it is thought by some, that a long - term advantage of out - crossing in nature is increased genetic variability that promotes adaptation and / or avoidance of extinction (see Genetic variability). </P>

Describe the process of fertilisation in flowering plants