Throughout his career Darwin wrote two kinds of books--those with a broad canvas, such as the evolution quartet, and those with a narrow focus, such as the treatise on barnacles. His interests shifted over the years from geology to zoology to botany. In these later works, however, he included theoretical interpretation, whereas his earlier works had contained mostly data. In On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects (1862) he demonstrated that plants exhibit complicated characteristics that are adaptive and that increase the survival of a species.
One such characteristic, for example, is cross-pollination (the mechanism by which pollen is transferred from one flower to another). In explaining the interdependence of bees and orchids, Darwin noted that flowers that are pollinated by the wind have little colour, while those that need to attract insects have brightly coloured petals and sweet-smelling nectaries. In The Different Forms of Flowers on Plants of the Same Species (1877) he observed that flowers in some species differ in the lengths of their anthers and styles, which is another adaptation for cross-pollination.
Experiments
Darwin experimented in his garden at Down House in Kent where he raised two large beds of Linaria vulgaris, one from cross-pollinated and the other from self-pollinated seeds, both of which he obtained from the same parent plant. He observed, "To my surprise, the crossed plants when fully grown were plainly taller and more vigorous than the self-fertilized ones."
He continued horticultural experiments for another 12 years on 57 species and described his results in The Effects of Cross and Self Fertilization in the Vegetable Kingdom (1876).Here he developed the theme that there are hereditary advantages in having two sexes in both the plant and animal kingdoms--to ensure cross-fertilization, which, as he knew from experiments, produced healthier, more vigorous offspring.