The older advocates of evolution sought for the causes of the process exclusively in the influence of varying conditions, such as climate and station, or hybridization, upon living forms. Even Treviranus has got no further than this point.
When selection is being made for several characters at the same time, and also in hybridization experiments, where it is important to have full records of the characters of individual plants and their progeny, " score cards," such as are used in judging stock, with a scale of points, are used.
A large number of horticultural varieties have been developed by hybridization, some of which have a variegated foliage.
The tobacco flower is fortunately perfectly self-fertile, and by enclosing the flowers of selected plants in paper bags, so as to exclude all possibility of hybridization, progeny true to the type of the mother plant can be obtained.
Hybridization can also be readily controlled in the case of tobaccos, and in this connexion it is useful to note that, if pollen is desired of some variety growing at a distance, it will retain its vitality for several weeks if kept perfectly dry, and so can readily be sent by post from one place to another.
From these, or rather from some of these, by cultivation and hybridization, have arisen the very numerous modern varieties.
Some of the most interesting results and many of the gardener's greatest triumphs have been obtained by hybridization, i.e.
It is obvious that hybridization differs more in degree than in kind from cross-fertilization.
The occurrence of hybrids in nature explains the difficulty experienced by botanists in deciding on what is a species, and the widely different limitations of the term adopted by different observers in the case of willows, roses, brambles, &c. The artificial process is practically the same in hybridization as in cross-fertilization, but usually requires more care.
- This may be called the natural means of increasing the number of any particular kind of plant, but it is to be remembered that we do not by that means secure an exact reproduction of the parent, especially in the case of plants raised or evolved in the course of generations by hybridization and selection.
If this is so, and the endosperm like the embryo is normally the product of a sexual act, hybridization will give a hybrid endosperm as it does a hybrid embryo, and herein (it is suggested) we may have the explanation of the phenomenon of xenia observed in the mixed endosperms of hybrid races of maize and other plants, regarding which it has only been possible hitherto to assert that they were indications of the extension of the influence of the pollen beyond the egg and its product.
Hybridization also is very uncertain, and is very difficult to carry out effectually; hence the method of propagating the best varieties by cuttings has been adopted, except in the case of those which do not strike readily, as in C. Ledgeriana, in which the plants are grown from the shoots of felled trees.