s features of their anatomy, transitional
characters between true fish and amphibia. Similarly, in the Permian
come mammal-like reptiles, that point also downward to the amphibia.
We find, therefore, the story told by the ovum written also in the
rocks.

Section 47. Now, when this fact of a common ancestry is considered,
it becomes necessary to explain how this gradual change of animal
forms may have been brought about.

Section 48. Two subcontrary propositions hold of the young of any
animal. It resembles in many points its parent. It differs in many
points from its parent. The general scheme of structure and the
greater lines of feature are parental, inherited; there are also novel and
unique details that mark the individual. The first fact is the law of
inheritance; the second, of variation.

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.

Section 50. The idea of this process of change may be perhaps
rendered more vivid by giving an imaginary concrete instance of its
working. In the jungles of India, which preserve a state of things wh