The author takes the case of a pair of animals, producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average survive to pro-create their kind.

This is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms.

He then shows that if a single individual were born, which varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances would be strongly against its survival.

Supposing it to survive and to breed, and that half its young inherited the favourable variation; still, as the Reviewer goes onto show, the young would have only a slightly better chance of surviving and breeding; and this chance would go on decreasing in the succeeding generations.

The justice of these remarks cannot, I think, be disputed.