amongst a population of Molluscs and Crustacea. The
toughest, the speediest, the most alert, the most retiring, or the least
conspicuous, will be the most apt to survive and breed. In hundreds or
thousands of generations there will be an enormous improvement in the
armour, the speed, the sensitiveness, the hiding practices, and the
protective colours, of the animals which are devoured. The "natural
selection of the fittest" really means the "natural destruction of the
less fit."

The only point assumed in this is that the young of an animal or plant
tend to differ from each other and from their parents. Darwin was
content to take this as a fact of common observation, as it obviously
is, but later science has thrown some light on the causes of these
variations. In the first place, the germs in the parent's body may
themselves be subject to struggle and natural selection, and not share
equally in the food-supply. Then, in the case of the higher animals (or
the majority of animals), there is a clear source of variation in
the fact that the mature germ is formed of certain elements from two
different parents, four grandparents, and so on. In the case of the
lower animals the germs and larvae float independently in the water,
and are exposed to many influences. Modern embryologists have found,
by experiment, that an alteration of the temperature or the chemical
considerable effect on eggs and larvae. Some recent experiments have
shown that such changes may even affect the eggs in the mother's
ovary. These discoveries are very important and suggestive, because the
geological changes which we are studying are especially apt to bring
about changes of temperature and changes in the freshness or saltiness
of water.

Evolution is, therefore, not a "mere description" of the procession of
living things; it is to a great extent an explanation of the procession.
When, however, we come to apply these general principles to certain
aspects of the advance in organisation we find fundamental differences
of opinion among biologists, which must be noted. As Sir E. Ray
Lankester recently said, it is not at all true that Darwinism is
questioned in zoology to-day. It is true only that Darwin was not
omniscient or infallible, and some of his opinions are disputed.

Let me introduce the subject with a particular instance of evolution,
the flat-fish. This animal has been fitted to survive the terrible
struggle in the seas by acquiring such a f