Then we have another important fact to consider, the principle of heredity or transmission of variations. If we grow plants from seed or breed any kind of animals year after year, consuming or giving away all the increase we do not wish to keep just as they come to hand, our plants or animals will continue much the same; but if every year we carefully save the best seed to sow and the finest or brightest coloured animals to breed from, we shall soon find that an improvement will take place, and that the average quality of our stock will be raised. This is the way in which all our fine garden fruits and vegetables and flowers have been produced, as well as all our splendid breeds of domestic animals; and they have thus become in many cases so different from the wild races from which they originally sprang as to be hardly recognisable as the same. It is therefore proved that if any particular kind of variation is preserved and bred from, the variation itself goes on increasing in amount to an enormous extent; and the bearing of this on the question of the origin of species is most important. For if in each generation of a given animal or plant the fittest survive to continue the breed, then whatever may be the special peculiarity that causes "fitness" in the particular case, that peculiarity will go on increasing and strengthening _so long as it is useful to the species_. But the moment it has reached its maximum of usefulness, and some other quality or modification would help in the struggle, then the individuals which vary in the new direction will survive; and thus a species may be gradually modified, first in one direction, then in another, till it differs from the original parent form as much as the greyhound differs from any wild dog or the cauliflower from any wild plant. But animals or plants which thus differ in a state of nature are always classed as distinct species, and thus we see how, by the continuous survival of the fittest or the preservation of favoured races in the struggle for life, new species may be originated. This self-acting process which, by means of a few easily demonstrated groups of facts, brings about change in the organic world, and keeps each species in harmony with the conditions of its existence, will appear to some persons so clear and simple as to need no further demonstration. But to the great majority of naturalists and men of science endless difficulties and objections arise, owing to the wonderful variety of animal and vegetable forms, and the intricate relations of the different species and groups of species with each other; and it was to answer as many of these objections as possible, and to show that the more we know of nature the more we find it to harmonise with the development hypothesis, that Darwin devoted the whole of his life to collecting facts and making experiments, the record of a portion of which he has given us in a series of twelve masterly volumes.