Section 49. Now the Parent or Parents, Since They Live and Breed,

must be more or less, but sufficiently, adapted to their conditions of living-- more or less fitted to the needs of life. The variation in the young animal will be one of three kinds: it will fit the animal still better to the conditions under which its kind live, or it will be a change for the worse, or it is possible to imagine that the variation-- as in the colour variations of domesticated cats-- will affect its prospects in life very little. In the first case, the probability is that the new animal will get on in life, and breed, and multiply above the average; in the second, it is probable that, in the competition for food and other amenities of life, the disadvantage, whatever it is, under which the animal suffers will shorten its career, and abbreviate the tale of its offspring; while, in the third case, an average career may be expected. Hence, disregarding accidents, which may be eliminated from the problem by taking many cases, there is a continual tendency among the members of a species of animals in favour of the proportionate increase of the individuals most completely adapted to the conditions under which the species lives. That is, while the conditions remain unchanged, the animals, considered as one group, are continually more highly perfected to live under those conditions. And under changed conditions the specific form will also change.